Fig. 27.—Sketch map showing a very early stage in the history of the Great Lakes when two relatively small lakes in front of the ice wall separately drained into the Mississippi River. (Drawn by the author from map by Taylor & Leverett.)
Fig. 27.—Sketch map showing a very early stage in the history of the Great Lakes when two relatively small lakes in front of the ice wall separately drained into the Mississippi River. (Drawn by the author from map by Taylor & Leverett.)
Fig. 28.—Lake Whittlesey stage of the Great Lakes history when the ice had retreated far enough to allow the eastern and western ice margin waters to join with a single outlet past Chicago. (Drawn by the author from a map by Taylor & Leverett.)
Fig. 28.—Lake Whittlesey stage of the Great Lakes history when the ice had retreated far enough to allow the eastern and western ice margin waters to join with a single outlet past Chicago. (Drawn by the author from a map by Taylor & Leverett.)
Fig. 29.—The Algonquin-Iroquois stage of the Great Lakes when their whole area was ice-free, and all their waters drained through the Mohawk-Hudson Valleys of New York into the Atlantic Ocean. (After Taylor, published by New York State Museum.)
Fig. 29.—The Algonquin-Iroquois stage of the Great Lakes when their whole area was ice-free, and all their waters drained through the Mohawk-Hudson Valleys of New York into the Atlantic Ocean. (After Taylor, published by New York State Museum.)
We shall now very briefly trace out the principal stages in the history of the Great Lakes during the final retreat of the vast ice sheet. This may best be done by the aid of maps which need only brief explanation. When the ice sheet had retreated far enough northward to uncover the very southern end of the Lake Michigan basin and a little beyond, a small glacial lake (Lake Chicago) developed against the ice wall. Its outlet was through the Illinois River and thence into the Mississippi. At the same time a larger glacial lake, held up by the ice wall, developed over the western part of the Erie basin and beyond. Its outlet was through the Wabash River. With further retreat of the ice a large lake (Whittlesey) covering considerably more than the area of Lake Erie developed, with outlet westward across Michigan into the enlarged Lake Chicago which continued to drain into the Illinois River. During a still later stage of ice withdrawal the remarkable set of three glacial lakes existed—LakesDuluth, Chicago, and Lundy. Each of these large lakes had its own outlet. Lake Duluth covered about half of the Lake Superior basin and drained through the St. Croix River into the Mississippi. Lake Chicago expanded to cover nearly all of the Michigan basin and continued to drain through the Illinois River. Lake Lundy covered not only more than the area of the Erie basin, but also considerable territory north of Detroit, and drained eastward alongside the ice lobe of the Ontario basin through the Mohawk and Hudson valleys of New York, and into the Atlantic Ocean. Just after the ice completely withdrew from the area now occupied by the Great Lakes, but still blocked the St. Lawrence Valley, the vast body of water called LakeAlgonquin more than covered the sites of the present Superior, Michigan, and Huron. At this time the land was distinctly lower toward the northeast than at present, causing the outlets to the west to be abandoned. The great Lake Algonquin poured its waters eastward through the Trent River channel of Ontario, Canada, into glacial Lake Iroquois, which was the great ancestor of Lake Ontario. Lake Iroquois, in turn, had its outlet eastward through the Mohawk and Hudson Valleys of New York. For part of the time at least, Lake Erie maintained a separate existence discharging into Lake Iroquois near Buffalo. During the Algonquin-Iroquois stage the combined area of all the lakes was notably greater than the present area of the Great Lakes. The volume of water discharged by the lakes through the Mohawk Valley of New York was doubtless greater than that which now goes over Niagara Falls. Gradually, as the St. Lawrence ice lobe waned, the outlet waters of the lakes began to move alongside the ice through the St. Lawrence Valley. Finally the ice withdrew far enough to free the St. Lawrence Valley and the waters of the Great Lakes region dropped to a still lower level, bringing about the Nipissing Great Lakes stage not greatly different from the present. East and northeast of the Lakes the land was low enough to allow tidewater (the so-called Champlain Sea) to extend through the Hudson, Champlain, and St. Lawrence Valleys, and possibly into the Ontario basin, as proved by the occurrence of marine beaches and fossils. The waters in the Erie and Ontario basins covered about the present areas, while the Nipissing Lakes, which covered a little more than the present areas of the three upper Great Lakes, had their outletthrough the Ottawa River channel into tidewater (Champlain Sea). Postglacial warping of the land has brought the whole region to the present condition.
Fig. 30.—Map showing next to the present stage of the Great Lakes history when the land was lower on the north and the upper (Nipissing) lakes drained through the Ottawa River Valley into an arm of the sea (Champlain Sea) which reached through the Champlain and Hudson Valleys. (After Taylor, published by New York State Museum.)
Fig. 30.—Map showing next to the present stage of the Great Lakes history when the land was lower on the north and the upper (Nipissing) lakes drained through the Ottawa River Valley into an arm of the sea (Champlain Sea) which reached through the Champlain and Hudson Valleys. (After Taylor, published by New York State Museum.)
Many lakes, including some remarkable ones, occupy basins which are directly due to movements of the earth’s crust—either faulting or warping. An example of a lake occupying part of a fault basin is the famous Dead Sea of Palestine. This lake lies in the lowest part of the Jordan Valley, which has geologically recently come into existence by the sinking of a long, narrow block of earth for several thousand feet between two great earth fractures (faults). The Dead Sea covers about 500 square miles and its surface lies about 1,300 feetbelow sea level, which makes it the lowest lake in the world. Almost equally remarkable is the fact that its depth is about 1,300 feet, so that the lowest part of the lake basin is 2,600 feet below sea level. The lake contains approximately 24 per cent salt, mostly common table salt, causing it to be a thick brine in which there is neither plant nor animal life—hence the name “Dead Sea.” At one time, probably just after the Ice Age, the lake was much larger and deeper, when it filled a considerable part of the Jordan Valley and had an outlet to the south. During the high-level stage the water was fresh, but gradually, as the climate became drier, evaporation was greater than intake, the outlet was abandoned, and the mineral matter (mostly chloride of magnesia and common table salt) carried by the streams in solution into the shrinking lake steadily accumulated until the high degree of salinity of the present time has been reached.
Great Salt Lake, Utah, is a remarkable lake whose history has been carefully studied. It occupies the lowest position of an extensive basin which, in turn, forms but part of the whole great district of Utah which has geologically recently sunk thousands of feet on the west side of the great fault already described as occurring along the western base of the Wasatch Mountains. At present the lake covers about 2,000 square miles, but its area fluctuates considerably. It is scarcely believable that this big lake has an average depth of only fifteen feet and a maximum depth of only fifty feet. It lies 4,200 feet above sea level, and it carries about 18 per cent salts in solution. Most abundant by far is common table salt, of which there are no less than 5,000,000,000 tons in solution. The waters also containabout 900,000,000 tons of other salts. Should the lake completely disappear by evaporation, these salts would be deposited. Allowing for cars 40 feet long and of 40 tons capacity, a train more than 1,000,000 miles long would be required to carry the salts. What has been the source of these salts? Great Salt Lake is not, as supposed by some, a remnant of an ocean once covering the region. Briefly, the explanation is as follows: At one time, when the climate was moister, the basin now only in part occupied by the lake was filled to overflowing with an outlet north into the Snake and Columbia rivers. That great body of water (called “Lake Bonneville”) covered nearly 20,000 square miles and its depth was about 1,000 feet deeper than now, the present depth being very small. Because it had an outlet that lake was, of course, fresh. Beaches and shore lines 1,000 feet above the present lake, and at various lower levels, are still wonderfully well preserved. When, due to climatic change, evaporation exceeded intake by streams, the outlet was cut off. But slowly, as the lake shrank, streams (especially the Jordan River) carried a little salt in solution, the percentage of salt increasing until the present stage has been reached. In a real sense, much of the salt was once in the sea, because it has been dissolved out of strata which accumulated under sea water long before the basin of Great Salt Lake came into existence.
Another famous lake, which also occupies part of a basin due to faulting, is Lake Tahoe in the Sierra Nevada Mountains, near Truckee, California. This lake, whose length is 21 miles, and width 12 miles, lies 6,225 feet above sea level. On almost all sides steep mountains rise several thousand feet above itswaters. Its great depth of 1,635 feet makes it, so far as known, the second deepest lake in North America, Crater Lake, Oregon, only outranking it. The water is exceedingly clear. An experiment some years ago showed that a white disk eight inches in diameter could actually be seen through a thickness of 216 feet of its water. “The statement sometimes made that “Tahoe is an old volcanic crater” is not true. The region about the lake shows evidences of volcanic activity of various kinds, and the lake waters themselves have probably been dammed at times by outpourings of lava. A lava flow appears to have temporarily filled the outlet channel below Tahoe City. The lake, however, lies in a structural depression—a dropped (fault) block in the earth’s crust.” (U. S. Geological Survey.)
The basin of the largest lake in the world—the Caspian Sea—has resulted from warping of the earth’s crust. It has an area of 170,000 square miles, a maximum depth of 3,200 feet, and its surface is about 90 feet below sea level. The composition of its water and some of its animal life indicate that it was once an arm of the sea. It has been detached or cut off by an upwarp of the land between it and the Black Sea region. If this great lake is a cut-off arm of the sea, with no outlet, how do we explain the fact that its salinity is much less than that of the ocean? Toward the north, where it is shallow and fed by so much river water, it is, in fact, almost fresh water. Even the southern one-half carries not over 1 per cent of salt. The explanation is that a steady current passes through a narrow passageway into a gulf or bay on its eastern side where evaporation is much greater than over the general surface of the Caspian. The salt is,therefore, gradually accumulating at the estimated rate of 350,000 tons per day in this gulf, while the sea itself is becoming fresher.
The basin of Lake Champlain, about 100 miles long, was occupied by tidewater geologically very recently (that is, since the Ice Age), but it has been cut off by uplift of the land on the north, since which time the waters of the lake have been completely rinsed out and freshened.
Many lake basins directly result from volcanic action. In many parts of the world lakes, usually of small size, occupy craters of volcanoes as, for example, in the Eifel region of Germany, the Auvergne district of France, and near Rome and Naples in Italy. Such a lake of exceptional interest fills part of the great crater, several thousand feet deep, which resulted from the explosion of Mt. Katmai, Alaska, in 1912. The water of this lake, more than a mile wide and of unknown depth, is hot.
One of the most unique and beautiful lakes of the world is Crater Lake in the Cascade Mountains of southern Oregon. It partly fills a great, nearly circular hole, six miles in diameter, with a maximum depth of about 4,000 feet, in the top of a mountain (Plate 11). The lake is over five miles in diameter and nearly 2,000 feet deep, making it the deepest in North America. Its surface is about 6,200 feet above sea level. Precipitous rock walls rising 500 to 2,000 feet completely encircle the lake, the main body of whose water is of a marvelous deep, sapphire-blue color, while the shallow portions around some of the shore are of emerald-green. Crater Lake has very little intake except direct rainfall and snowfall, and its water is fresh. The great hole was not produced by an explosion like that of Katmai,but rather by the sinking of the top of a once much greater mountain. That the mountain was once about the size and shape of Mt. Shasta is proved by the fact that deep glaciated valleys lead up the slopes and end abruptly at the very rim of the present mountain. Obviously these valleys were scoured out in recent geologic time by glaciers whose sources were several thousand feet up on a former cone-shaped mountain. That the mountain top sank rather than exploded is proved by the absence of volcanic débris over the sides and base of the mountain.
Still another way by which lakes are formed by volcanic action is by streams of lava blocking valleys. The famous Sea of Galilee in Palestine was thus formed by a stream of lava, which geologically recently flowed down from the east into the Jordan Valley and across it, where it cooled to form a dam ponding the waters of the Jordan River. Because the river flows through the lake, its water is fresh. One of the most remarkable facts about this lake is that its surface lies nearly 700 feet below sea level. A number of lava-dam lakes are known in the Sierra Nevada and Cascade Mountains.
A very interesting case of a lake basin, formed by cutting off an arm of the sea without any movement of the earth’s crust, is the Salton Sink of southern California. This basin, many miles long and wide, lies below sea level, its lowest point being 287 feet below tide. The Gulf of California formerly reached much farther north and into California where it covered the site of the Salton Sink. Gradually the Colorado River, always loaded with sediment, built a broad delta deposit right across the gulf, the northern end of which thus becamecut off, leaving a big salt lake. But the river flowed into the gulf, while in the dry climate the evaporation was great enough to gradually dry away the salt lake. This was the condition of things until 1904, when much of the river at a time of flood got out of control and, following the general course of a great irrigating canal, it flowed for several years into the lowest part of the Salton Sink, partly filling it to form a lake 45 miles long, 17 miles wide, and 83 feet deep. Since 1907 the lake has been notably decreasing in size, and it may entirely disappear.
Other ways by which lakes, mostly relatively small ones, may develop are by landslides blocking valley drainages; by streams cutting across winding curves leaving so-called “oxbow lakes” which are common, for example, along the lower Mississippi River; by wave and wind action along shores of lakes or sea; by filling so-called “sink holes” which result from dissolving or falling in of roofs of caves; and by beavers through whose industry dams are built across valleys or streams.
Some of the most common ways by which lakes may be destroyed are the following: by being filled with sediment carried in by streams, or by vegetation, or by both; by cutting down outlets; by evaporation due to a change in climate; by removal of the ice dam in certain types of glacial lakes; and by movements or warping of the earth’s crust.
CHAPTER XII
HOW THE EARTH MAY HAVE ORIGINATED
T
THE problem of the origin of the earth is essentially astronomical rather than geological, because geological history is considered to have begun when common earth processes, such as erosion, deposition, and transportation of sediments, etc., were brought into play. It is quite certain, however, that the earth in its pregeologic state gradually merged into its geological condition. For this reason the geologist is interested in the more important doctrines or hypotheses which have been put forth to account for the origin of the earth. In fact, one of the few hypotheses which must be taken seriously is largely the work of a geologist. The most acceptable hypothesis not only best satisfies the facts regarding the earth’s astronomical relationships, but also best harmonizes with our knowledge of the oldest known rocks and their history.
Since the problem of the origin of the earth is an essential part of the problem of the origin of the solar system, the following well-known facts should be clearly in the mind of the reader. Eight planets, including the earth, revolve in nearly circular paths around the central sun, whose diameter is 866,000 miles. The radius of the solar system is at least 2,800,000,000 miles, this being the distance of theoutermost known planet (Neptune) from the Sun. Neptune requires 164 years for a trip around the sun, while the earth, which averages about 93,000,000 miles from the sun, makes its circuit once a year. The planets all revolve around the sun in the same direction, and in nearly the same plane. The sun and all eight planets rotate on their axes in the same direction, the earth’s rotation being accomplished every twenty-four hours. Most of the planets have one or more smaller bodies called satellites revolving about them, such as Earth, with its one satellite (the moon), and Saturn, with its eight satellites, etc. It is well known that this solar system is only a very small part of the vast universe, as shown by the facts that no star is nearer the earth than several trillion miles, and that some stars are so far away that light traveling at the rate of 186,000 miles per second requires a thousand years to reach the earth!
Toward the end of the eighteenth century the famous nebular or ring hypothesis was set forth by the astronomer named Laplace. This assumes an original very hot incandescent mass of gas spheroidal in shape and greater in diameter than the present solar system. This mass rotated in the direction of rotation of our sun and its planets. Loss of heat by radiation caused the mass to shrink, and this in turn not only made it rotate faster, but also caused the centrifugal force (i.e., the force whose direction was from the center) in its equatorial portion to gradually become stronger. Finally a time came when the force of gravity (i.e., the force whose direction was toward the center) and the centrifugal force became equal and a ring was left (not thrown) off, while the rest of the mass of gas continued to shrink. After a time the materialof the ring collected to form the outermost planet. The other planets were similarly formed from other rings which were left off as contraction of the great mass of gas went on. The sun represents the remainder of the great mass of rotating gas.
What is the bearing of this nebular hypothesis upon the early geological history of the earth? According to the hypothesis the earth must once have been much more highly heated and larger than now. It condensed to a liquid and then it cooled enough to permit the formation of a solid crust over a liquid interior. It then had a hot dense atmosphere containing all the water of the earth in the form of vapor, and this atmosphere steadily became thinner due to absorption by the earth. When the pressure and temperature conditions became favorable, much of the water vapor condensed to form the ocean and the atmosphere gradually changed to its present condition. According to this view the oldest rocks of the earth must have been igneous because they resulted from the solidification of the outer part of the molten globe.
Within recent years certain serious objections to the nebular hypothesis have been raised, and Chamberlin and Moulton have formulated the planetesimal or spiral hypothesis as an attempt at a more rational explanation of the origin of the solar system. Some of the objections to the older doctrine are that among the many thousands of known nebulæ in the universe very few only are of the Laplacian or ring type, while spiral forms are abundant. Spectroscopic study shows that the nebulæ are not gaseous, but made up of either liquid or solid particles, and that the leaving off of rings would necessitate the assumption of an intermittent process which could scarcely have operated under the conditions of the hypothesis.
Plate 9.—(a)Molten Lava Flowing Over a Cliff Into Water in the Hawaiian Islands.(After Diller, U. S. Geological Survey.)
Plate 9.—(a)Molten Lava Flowing Over a Cliff Into Water in the Hawaiian Islands.(After Diller, U. S. Geological Survey.)
Plate 9.—(b)Dikes of Granite (Light Gray) Cutting an Old Dark Rock.While the granite on the right was being forced in molten condition upward into the earth’s crust, tongues of it (dikes) were sent off into the adjacent rock. (Photo by Howe, U. S. Geological Survey.)
Plate 9.—(b)Dikes of Granite (Light Gray) Cutting an Old Dark Rock.While the granite on the right was being forced in molten condition upward into the earth’s crust, tongues of it (dikes) were sent off into the adjacent rock. (Photo by Howe, U. S. Geological Survey.)
Plate 10.—(a)Lassen Peak, Northern California, in Eruption August 22, 1914.The great cloud of steam and volcanic ash rose several miles. This is the only active volcano in the United States proper, and it is now included in Lassen Volcano National Park. (By permission of R. E. Stinson, Red Bluff, Cal.)
Plate 10.—(a)Lassen Peak, Northern California, in Eruption August 22, 1914.The great cloud of steam and volcanic ash rose several miles. This is the only active volcano in the United States proper, and it is now included in Lassen Volcano National Park. (By permission of R. E. Stinson, Red Bluff, Cal.)
Plate 10.—(b)Devil’s Tower, Wyoming.This great mass of rock was forced in molten condition through strata which, because of their weakness, have been eroded away all around the hard igneous rock. This is probably the core or neck of a former volcano. (Photo by Darton, U. S. Geological Survey.)
Plate 10.—(b)Devil’s Tower, Wyoming.This great mass of rock was forced in molten condition through strata which, because of their weakness, have been eroded away all around the hard igneous rock. This is probably the core or neck of a former volcano. (Photo by Darton, U. S. Geological Survey.)
Fig. 31.—Diagram showing the origin and character of a spiral nebula according to the planetesimal hypothesis of the origin of the solar system. (Modified after Moulton.)
Fig. 31.—Diagram showing the origin and character of a spiral nebula according to the planetesimal hypothesis of the origin of the solar system. (Modified after Moulton.)
Anything like a full understanding of the planetesimal hypothesis would be difficult to obtain, and, in the brief space at our disposal, we shall attempt to make clear only a few of the salient points. According to this hypothesis the solar system was, during a previous stage of its evolution, a great, flat, spiral nebula, made up of finely divided solid or possibly liquid particles called planetesimals, among which were scattered some larger “knots” or masses. Each tiny particle and larger mass or knot is considered to have traveled in its own particular orbit or path about a central very large mass—the future sun. It is even suggested that the spiral nebulaoriginated by disruption of one star by a swift-moving passing star. Each disrupted particle and large mass at first started straight for the large passing star, but because of change of position of the latter the particles and larger masses were gradually pulled around so that their paths curved into spirals. Because of crossing of paths, the larger masses or knots gradually increased in size by accretion of the small particles or planetesimals. Meteors (so-called “shooting stars”) which now strike the earth are thought to be disrupted materials still being gathered in, though very slowly at present. After the passing star got well out of range, the spiral paths of the disrupted masses gradually changed to nearly circular, due to a wrapping-up process around the central body (sun) which then controlled the movements of the both larger masses (future planets) and small masses (planetesimals).
Let us now inquire briefly into the bearing of this planetesimal hypothesis upon the early geological history of the earth. According to this doctrine the earth was never in the form of a highly heated gas, nor was it ever necessarily hotter than now. Instead of beginning as a much larger body which has gradually diminished in size, the earth steadily grew, up to a certain stage, by ingathering of planetesimals. Increase in size caused the force of gravity to increase and this caused not only steady contraction of the earth’s matter, but also a development of greater internal heat. The earth has been getting smaller ever since the force of compression has predominated over the building-up process, because of the diminishing supply of planetesimals. Due to steadily increasing internal pressure andheat the various gases, including water vapor, have been driven out of the earth to form an atmosphere which has gradually become larger and denser. After sufficient accumulation of water vapor, condensation and rainfall took place; the waters of the earth began to gather to form the oceans; and the ordinary geologic processes of erosion and deposition of strata were initiated. According to this view stratified rocks could have been formed very early in the history of the earth, and in this connection it is interesting to note that the oldest known rocks are actually of sedimentary origin.
CHAPTER XIII
VERY ANCIENT EARTH HISTORY
(Archeozoic and Proterozoic Eras)
W
WE shall now consider the older rocks of the Earth, including those of Archeozoic, Proterozoic, and Paleozoic ages. What are the salient points in the very early history of the earth (not including the evolution of organisms) shown by these very ancient rocks? Beginning with the oldest known rocks, it will be our purpose to trace out the principal recorded events of earth history in the regular order of their occurrence. As in human history, so in earth history the recorded events of very early times are fewest and most difficult of all to understand. In spite of this difficulty it is best to begin with the oldest known rocks or, as Le Conte has said, “to follow the natural order of events. This has the great advantage of bringing out the philosophy of the history—the law of evolution.” Because of limitation of space we shall give special attention to the physical history of North America, but the general principles brought out apply almost equally well to the other continents.
The Archeozoic rocks contain the earliest known records of geological history, or, in other words, the oldest recorded ordinary geological processes such as weathering and erosion, deposition of strata, igneousactivity, etc. Although we are here dealing with the most obscure records of any great rock system, partly because the rocks have been so profoundly altered (metamorphosed), and partly because of the absence of anything like definitely determinable fossils, it is, nevertheless, true that certain very important conclusions have been reached regarding this very ancient geological era.
Among the very oldest of all known rocks of North America are the Grenville strata, so named from a town in the St. Lawrence Valley. In fact, no rocks elsewhere in the world have been proved to be more ancient. The Grenville series consists of a great mass of sediments (strata)—original muds, sands, and limes—which were deposited layer upon layer under water (Plate 12). The widespread extent and character of the series in southeastern Canada and the Adirondacks, and more than likely far beyond these limits, make it certain that the Grenville strata were accumulated on the bottom of a relatively shallow sea very much as sediments are now piling up on shallow sea bottoms. Thus, the most ancient definitely known condition of the region where the Grenville strata are exposed was an expanse of the sea covering the whole area. Wherever, in other parts of the world, the Archeozoic rocks have been studied, stratified rocks also seem to be the very oldest which are recognizable, but up to the present time no such rocks have been proved to be any older than, or even as old as, the Grenville series.
It may occur to the reader to ask, how long ago did the Grenville ocean exist? There are grave difficulties in the way of answering this question in terms of years since we have nothing like an exactstandard for such a measurement or comparison. Although we must concede that not even approximate figures can be given, it can, nevertheless, be demonstrated by several independent lines of reasoning that the time must be measured by at least tens of millions of years, a very conservative estimate of the minimum time which must be allowed being about 50 million years. In any case, the time is so utterly inconceivable to us that the important thing to bear in mind is that the great well-known events of earth history, which have transpired since the existence of the Grenville ocean, require a lapse of many millions of years, as shown by revolutionary changes in geographic and geologic conditions such as the long periods of erosion, the enormous accumulations of sediment, the repeated spreading out and disappearance of sea water over many portions of the earth, and the building up and tearing down of great mountain ranges at various times. The ideas here expressed will be much better appreciated by the reader after following through the salient points in the history of North America as set forth in the succeeding pages.
Again, the reader may ask, by what line of reasoning do we conclude that these stratified rocks are so exceedingly ancient? All rocks of Archeozoic Age, including strata as well as certain younger igneous rocks (seebelow), invariably occupy a basal position in relation to all other rock systems. They constitute a complex lot of crystalline metamorphic rocks, combining certain characteristics which lie below the base of the determined sedimentary succession. Where rocks with the characteristics of the Archeozoic are separated from the oldest Paleozoic(Cambrian) strata by the great sedimentary or metamorphic system known as the Proterozoic (see below), we may be sure that we are dealing with Archeozoic rocks. If the series of rocks in question belongs in the Archeozoic system, all that remains is to determine its age position in that system. This can usually readily be done because wherever they have been studied the Archeozoic rocks may be subdivided into two groups of rocks, a sedimentary and an igneous. Where the igneous rocks, mainly granites and related types, occur associated with the sedimentary rocks (e.g., Grenville), they very clearly were forced or intruded, while molten, into the sedimentary rocks, thus proving these latter to be the older.
Since the Archeozoic strata of the Adirondack Mountains, southeastern Canada, and also all, or nearly all, other known districts are mostly badly disturbed, tilted, and more or less bent or folded, and since neither top nor bottom of the piles of strata has ever been recognized as such, it is impossible to give anything like an exact figure for the thickness of the series. Continuous successions of strata have, however, been observed in enough places to show that they were commonly deposited layer upon layer to a thickness of at least some tens of thousands of feet. A thickness of over 100,000 feet has been reported from southeastern Canada. The clear implication is that the Archeozoic sea which received sediments must have existed for a vast length of time which must be measured by at least some millions of years, because in the light of all our knowledge regarding the rate of accumulation of sediments a very long time was necessary for the piling up of such thick masses of strata. It doesnot, however, necessarily follow that the Grenville ocean was many thousands of feet deep where deposition took place. In fact, the very character of the original sediments (muds, sands, and limes) clearly indicates that the Archeozoic sea in which they accumulated was, for most part at least, of shallow water because such sediments have rarely, if ever, been carried out into an ocean of deep water. The great ocean abysses of to-day are not receiving any appreciable amount of land-derived sediment. Thus we are forced to conclude that in Archeozoic time, as well as many times in later ages, the shallow sea bottom gradually sank while the sediments accumulated. Even more conclusive proof of such subsidence has been obtained from the study of so-called “folded” mountain ranges of Paleozoic and later time, an excellent example being the Appalachian Range.
Having established the sedimentary origin and great antiquity of the Grenville series, we are led to the interesting and important conclusion that these oldest known rocks are not the most ancient which ever existed, because the Grenville strata must have been deposited layer upon layer, upon a floor of still older rocks. If such still older rocks are anywhere exposed to view, they have never been recognized as such. Again, the fact that the most ancient known rocks were deposited under water carries with it the corollary that there must have been lands at no great distances from the areas of deposition because, then as now, such sediments as muds and sands could have been derived only from the wear or erosion of lands, and have been deposited in layers under water adjacent to those lands. But we are utterly in the dark regardingany knowledge of the location or character of such very ancient lands.
The most ancient known strata, as we see them to-day, do not look like ordinary sediments such as shales, sandstones, and limestones. They have been profoundly changed from their original condition, that is to say, they have undergone metamorphism. The Archeozoic strata now exposed to view were formerly buried at least some miles below the earth’s surface, the overlying younger rocks having since been removed by erosion through the millions of years of time. Far below the earth’s surface, under conditions of relatively high temperature, pressure, and moisture, the materials of the strata were completely crystallized into various minerals. The surfaces of separation of the very ancient layers of sediment are still usually more or less clearly present (Plate 12). Original limestone has been changed into crystalline limestone or marble; sandstone has been changed into quartzite, and shale, sandy shale, and shaly sandstone have been changed into various schists and gneisses.
In western Ontario there are also stratified rocks (called the Keewatin series) which seem to be of about the same age as the Grenville strata farther east. A point of special interest in connection with the Keewatin strata is the presence of layers of lava in portions of the series, thus proving that molten rock materials were poured out on the earth’s surface during the most ancient known era of the earth’s history.
After the accumulation of the very ancient Archeozoic sediments igneous activity took place on grand scales when great masses of molten rock were forced (intruded) into the sediments from below. Massesof molten materials are known to have been thus intruded at several different times, but of these the most common by far cooled to form a great series of granite and closely related rocks. The general effect was to break the old strata up into patches or masses of varying sizes as clearly shown by the present distribution and modes of occurrence of these igneous rocks. In most cases the strata were pushed aside by, or tilted or domed over, the upwelling molten floods—in many cases the molten materials were, under great pressure, intimately forced or injected into the strata; numerous large and small masses of strata were caught up or enveloped (as inclusions) in the molten floods; in some cases there was local digestion or assimilation of the strata by the molten materials, while in still other places large bodies of strata seem to have been left practically intact and undisturbed. Such igneous rocks, which are very widespread, are all of the plutonic or deep-seated types; that is, they were never forced up to the earth’s surface like lavas, but they solidified at considerable depths (at least some thousands of feet) below the surface. We see them exposed to-day only because a tremendous amount of overlying rock materials has been removed by erosion. These igneous rocks are generally easily distinguished from the old sediments of Grenville age because of their more general homogeneity in large masses, and their lack of sharply defined bands or layers of varying composition. The fact that the minerals have always crystallized to form medium to coarse-grained rocks shows that these rocks solidified under deep-seated conditions, since it is well known that surface flows (lavas) are much finer grained commonly with more or less of the rock not crystallizedat all. Slow cooling under great pressure favors more complete crystallization with growth of larger crystals.
As we have just learned, the very character and structure of the Archeozoic rocks now exposed to view show conclusively that they were formerly deeply buried, and the inference is perfectly plain that the overlying rock materials were removed by erosion. Profound erosion of any land mass means that the land must have stood well above sea level, and thus we come to the important conclusion that the great mass of Archeozoic rocks (both strata and igneous rocks) were upraised well above sea level. Just when the uplift occurred cannot be positively stated, but in every region where the matter has been studied it took place before the strata of the next geological era began to deposit as shown by the fact that such later strata rest upon the profoundly eroded surface of the Archeozoic rocks. Such an erosion surface, called an “unconformity,” marks a gap in the geological record of the district where it occurs. There is much to support the view that the uplift was concomitant with the great igneous intrusions, especially the granite. It is reasonable to believe that the same great force which caused the welling up of such tremendous bodies of liquid rock into the earth’s crust might easily have caused a decided uplift of a whole large region, but even so the process must have been geologically slow. In regard to the height of those ancient lands, the character of the topography, and the drainage lines we are as yet utterly in the dark. The fact that many thousands of feet in thickness of materials were removed by erosion to expose the once deeply buried rocks, does not necessarily implythat the lands at any time had great height, because it is possible that while elevation slowly progressed, much material was steadily removed by erosion. In the light of our knowledge of the origin and growth of mountain ranges of later time there is little doubt that at least some of the Archeozoic lands were raised to such mountain heights.
Thus far in our study of the Archeozoic rocks attention has been mainly directed to southeastern Canada and the Adirondack mountains, where careful studies have been made. In all parts of the world where the most ancient known (Archeozoic) rocks have been studied in detail the same general principles apply. Particular attention has been given to the Archeozoic rocks south of Lake Superior, and in the Piedmont Plateau of the eastern United States. In the accompanying map Archeozoic rocks are widely exposed to view within the areas shown in black. It has been estimated that Archeozoic rocks appear at the surface over about one-fifth of the land area of the earth. Where they are not at the surface it is believed that they everywhere exist under cover of later rocks. In other words, Archeozoic rocks are considered to be almost universally present either at or under the earth’s surface. This is true of the rocks of no other age. Special mention should be made of the fine exhibitions of Archeozoic rocks in Scandinavia and the Highlands of Scotland.
Fig. 32.—Map showing the surface distribution of Archeozoic and Proterozoic rocks in North America. (Redrawn by the author after U. S. Geological Survey.)
Fig. 32.—Map showing the surface distribution of Archeozoic and Proterozoic rocks in North America. (Redrawn by the author after U. S. Geological Survey.)
All known evidence leads us to the remarkable conclusion that the climate of much, or possibly all, of Archeozoic time was not fundamentally different from that of to-day. There must have been weathering of rocks, rainfall, and streams much as at present as proved by the character and composition of the stratified rocks which formed in that remote era. The presence of graphite (“black lead”) in crystalline flakes scattered through many of the strata shows that the climate must have been favorable to some form of life, because graphite thus occurring quite certainly represents the remains of organisms, this matter being more fully discussed in a succeeding chapter. In passing it may be stated that climatic zones were then probably scarcely if at all marked off, as they quite certainly were not even during Paleozoic time. One of the great contributionsof geology to human knowledge is that during the tens of millions of years from Archeozoic times to the present the earth’s climate has undergone no fundamental change or evolution. In the earlier ages there was greater uniformity of climate over the earth, and, during known geologic time there have been rather localized relatively minor fluctuations giving rise to glaciers, deserts, etc., but there has been no real evolution of climate at all comparable to the marvelous evolution of organisms—both animals and plants.
We shall now turn our attention briefly to a consideration of the second great subdivision of geologic time—the Proterozoic era. Rocks of Proterozoic Age comprise all of those which were formed after the Archeozoic rocks and before the deposition of the earliest Paleozoic (Cambrian) strata, these latter being rather definitely recognizable because they contain fossils characteristic of the time. Cambrian strata are, in fact, the oldest rocks which contain anything like an abundance of fossils, so that the separation of rocks of either Archeozoic or Proterozoic Age from the earliest Paleozoic is seldom difficult. But how may we separate the Proterozoic rocks from the Archeozoic? Fossils afford us no aid whatever, because no determinable fossils have been found in rocks as old even as the earlier Proterozoic. The two great groups of very ancient rocks do, however, show a number of differences which must be considered together. Thus, igneous rocks distinctly predominate in the Archeozoic, while stratified rocks predominate in the Proterozoic. All Archeozoic strata are thoroughly metamorphosed (changed from their original condition), while large masses of the Proterozoic strata are onlymoderately metamorphosed, or even unaltered, and therefore look much like ordinary strata of later ages. Archeozoic rocks have almost invariably been notably deformed by more or less folding, tilting, etc., while the Proterozoic rocks show relatively much less deformation. Another important criterion is the fact that the Proterozoic rocks, wherever they have been studied in relation to the Archeozoic rocks, always rest upon a profoundly eroded surface of the latter, that is, an unconformity separates the two great sets of rocks. This erosion surface is of still further interest because it is the very oldest one known, none having been recognized within the Archeozoic group itself. Even where the Proterozoic strata have been considerably metamorphosed and deformed, this old erosion surface may be recognized, and if the rocks below that surface possess the characteristics of the Archeozoic rocks as described above, the two great very ancient rock groups may be distinguished. One of the triumphs of geology during the last 25 to 30 years has been the recognition of the great rock group (Proterozoic) between the Archeozoic and Paleozoic, thus bringing to light the records of an era which lasted many millions of years.
The length of time represented by the Proterozoic era is by many believed to have been fully as long as all succeeding eras—Paleozoic, Mesozoic, and Cenozoic—combined. Twenty million years would be a very conservative estimate for the duration of the era. What is the nature of the evidence as recorded in the rocks which lead us to conclude that the Proterozoic era lasted such a vast length of time? The great thickness of Proterozoic strata (over 30,000 feet in the Lake Superior region), in the lightof what we have already learned regarding the present rate of wear (erosion) of lands and deposition of the eroded materials under ordinary conditions, clearly implies millions of years of time for their accumulation. But the Proterozoic strata as we now see them are in most places not a continuous pile, that is they were not accumulated layer upon layer without notable interruption. Thus, the thick Proterozoic group of the Lake Superior region has been divided into four distinct, mainly sedimentary series separated from each other by erosion surfaces (unconformities). Each erosion surface represents a long time when the area was elevated and underwent profound wear before the next series of strata accumulated on the worn surface. That such times of erosion were geologically long is proved not only by the profound alteration (metamorphism) of one set of strata before another accumulated, but also by the fact that granite, which, as we have learned, is never exposed except where much overlying material has been eroded, actually formed parts of surfaces of earlier Proterozoic rocks upon which later ones were deposited. In the Lake Superior region there are not only three great erosion surfaces (unconformities) within the Proterozoic group, but also one at the base separating it as a whole from the Archeozoic group, and another at the top separating it from the Paleozoic group. It is, therefore, fair to conclude that the amount of time (millions of years) represented by these great erosion intervals was fully as great as the time needed for deposition of the existing Proterozoic strata.
In the Lake Superior region the older Proterozoic strata are nearly all more or less folded and altered (metamorphosed), and they have been intruded byconsiderable bodies of molten rock, mostly granite. The later Proterozoic strata have been much less deformed and in many cases they are practically unaltered. In this region a very remarkable event took place in late Proterozoic time. This was volcanic activity on a grand scale. We may gain some idea of the stupendous and long-continued volcanic outpourings from the fact that, based upon actual measurements of thickness, lava sheets, averaging about 100 feet thick, poured out one upon another until a pile about six miles high had accumulated.
In parts of the Grand Canyon of the Colorado tilted Proterozoic strata may be seen resting upon the profoundly eroded surface of the Archeozoic rocks of the inner gorge. The Proterozoic strata, 12,000 feet thick, consist of practically unaltered sandstones, shales, and limestones, associated with some layers of basaltic lava. An erosion surface (unconformity) separates the whole group into two distinct series, and the group is separated from the overlying nearly horizontal Paleozoic (Cambrian) strata in the walls of the Canyon by another erosion surface.
More recently the Proterozoic strata so finely displayed in the Rocky Mountains of Montana and southern Canada have been studied. These strata, at least two or three miles thick, are mostly unaltered sandstones, shales, and limestones, associated with some metamorphic and igneous rocks. As usual, these strata rest upon the eroded Archeozoic. They were more or less upturned and folded before deposition of the succeeding Paleozoic strata. Satisfactory subdivisions have not yet been worked out.
In North America most of the areas shown on the accompanying map contain more or less Proterozoicrocks. Rocks of this age are known to some extent in all continents where their general relationships seem to be much like those of North America. They have perhaps been most carefully studied in Scandinavia and the Highlands of Scotland, where the strata portions are about two miles thick.
The climate of Proterozoic time must, for most part, have been about like that of to-day except, of course, for its much greater uniformity over the earth. About a dozen years ago very typical glacial deposits were discovered within the early Proterozoic rocks of western Ontario, Canada. A climatic condition favorable for the development of glaciers so early in the history of the earth is, to say the least, directly opposed to an idea (based upon the nebular hypothesis) long held that the climate of early geologic time must have been much warmer than that of the present.
CHAPTER XIV
ANCIENT EARTH HISTORY
(Paleozoic Era)