Chapter 11

Fig. 65.—An early type of bird with teeth. This bird grew to a height of about nine inches in Cretaceous time, millions of years ago. (Restored by Marsh.)

During the early part of the Cenozoic era birds became still more advanced and numerous, with many modern groups represented. Some of the more primitive types were, however, still left over during the Tertiary, as, for example, a toothed bird, in which the teeth were merely dentations of the bill, thus being the most degenerate of all types of tooth structure.

Mammals comprise the highest class of all animals. They are, of course, all warm blooded and characterized by suckling their young. So far as known, mammal life began in the early Mesozoic era as a branch of primitive reptiles, but they made little progress throughout the era when they occupied a very subordinate position in the animal world. They were few in number, small, and primitive in structure. There is no evidence for the Mesozoic existence of any of the higher forms of mammals, that is, those which give birth to well-formed young which are prenatally attached to the mother by the so-called placentum. “During the eons of the Mesozoic, from late Triassic time until its close, the mammals (including the remote progenitors of humanity) were in existence, but held in such effective check (by reptiles) that their evolutionary progress was practically insignificant. This curb is strikingly illustrated by the wonderful series of tiny jaws and teeth of these diminutive creatures found in the Comanchian (early Cretaceous) of Wyoming, in actual association with the single tooth of a carnivorous dinosaur, many times the bulk of the largest mammalian jaw. The removal of this check resulted (in the Tertiary period) in the speedy evolution of the archaic mammals.” (Schuchert.)

The phenomenal development of mammals during the Tertiary period forms one of the most wonderful chapters in the whole evolution of organisms. Even very early in the Tertiary, many important higher (placental) types of mammals had evolved, and the simpler, more primitive Mesozoic forms became very subordinate. By the close of the Tertiary the higher types of mammals had become marvelously differentiated into most of the present-day groups or types. A very significant feature of the evolution was the steady increase in relative size of brain. The vast numbers of fossil skeletons and bones of mammals found in Tertiary strata is scarcely believable. In our brief discussion we can do no more than describe a few representative examples of the Cenozoic evolution of mammals.

The great diversity of modern placental animals may be suggested by a few examples, as the tiger, dog, horse, camel, elephant, squirrel, hedgehog, whale, monkey, and man. Forms like these, traced back through their ancestors to the very early part of the Tertiary period, gradually become less and less distinct until they cannot be at all distinguished as separate groups, but rather there are ancestral generalized forms which show combinations of features of the later groups. Those early Tertiary generalized placental mammals had four feet of primitive character, with five toes on each foot; the whole foot, which from toe to heel touched the ground, was not adapted to swift running; the teeth were simple (primitive) in type and of full original number (forty-four); the toes were supplied with nails which were about intermediate between real claws and hoofs in structure;and the brain was relatively much smaller and simpler in structure than in most modern mammals.

Fig. 66.—Chart showing the main features of interest in the evolution of the horse family through several million years of the present (Cenozoic) era of geologic time. (After Matthew, American Museum of Natural History.)

The history of the horse family furnishes an excellent illustration of certain evolutionary changes among mammals. Skeletons of many species, ranging from the early Tertiary to the present, have been found in remarkable state of preservation representing every important change in the history of the horse family. A study of the chart will make clear some of the most striking changes which have taken place. The oldest member of the horse family represented on the chart was about the size of a small fox, with four toes and a degenerated fifth toe (splint) on the front foot, and three toes and splint on the hind foot. Since the chart was made a still more primitive form, even more closely resemblingthe original five-toed ancestor, has been found. Gradually the middle toe enlarged, while the others disappeared except the two splints or very degenerate toes still left in the modern horse. Increase in size of the animal and brain capacity accompanied these changes. Also the teeth underwent notable change, and two originally separate bones (radius and ulna) of the foreleg became consolidated into a single stronger bone.

The even-toed hoofed mammals of to-day, like the deer, pig, and camel, are also the product of evolution much like that of the horse, except that two of the original five toes have been equally developed, while the others have either greatly degenerated, as in the pig, or disappeared entirely, as in the camel.

The elephants, or trunk-bearing animals, illustrate a very different kind of evolution. They seem to have reached their climax of development in the late Tertiary when they grew to be as much as 14 feet high, and were more abundant and widespread over the earth than at any other time. The modern elephant, like the horse, has been traced back through many intermediate forms to its primitive early Tertiary ancestry. Some of the most important evolutionary changes took place in the head portion. The trunk is a highly developed form of snout, the earliest form of which was much like that of the modern tapir. The tusks are highly specialized and elongated teeth. During the earlier history the chin was very long and supported short tusks, so that there were then four tusks.

Carnivorous mammals, like tigers and wolves, and gnawers, like rats and squirrels, may also be traced back to generalized early Tertiary types.

Another kind of evolution is well illustrated by certain mammals which, even in early Tertiary time, so thoroughly adapted themselves to a water environment as to become whales, porpoises, etc.

Fig. 67.—Comparison of feet of monkeys and man.

The primates include the highest group of all Vertebrates, and therefore of all animals. Monkeys, apes, and man belong to the primates. There is no evidence whatever for the appearance of even the simplest and most primitive forms before the opening of the Cenozoic era, but even very early in Tertiary time, lemurs and primitive types of monkeys existed. Later in the Tertiary true monkeys and apes were common, and by the close of the period some apes were highly enough developed to strongly resemble certain of the oldest and most primitive types of man. We have, however, no positive knowledge of the existence of man in even the latest Tertiary. In the light of much evidence in regard to the antiquity of man, it seems improbable that true human fossils will ever be found in rocks older than the Quaternary, though if we are willing to descend (far enough in the human scale toward apes) it is not unlikely that man-apes may be discoveredin very late Tertiary rocks. The difficulty comes in the classification. Where are we to draw the line between the higher apes and the lowest forms of man? But this very difficulty is one of the strongest arguments in favor of the organic evolution of man because practically all intermediate forms between true man and certain other high-grade primates are known from the strata. The following tabular summary of the geological history of man is based upon the work of most of the ablest students of the subject.

Of the early ancestral forms, that is, those which were rather distinctly man-apes, two will be very briefly referred to. One of these, known asPithecanthropus erectus, was a remarkable creature whose partial skeleton, consisting of the upper part of a skull, lower jaw, several teeth, and a thigh bone, was found in early Quaternary deposits in Java in 1891. It was certainly a man-ape or possibly ape-man of low order, about 51/2feet high. The skull has a low crown, very receding forehead, and prominent brow ridges, but the brain capacity is 850 cubic centimeters, as compared to 500 cubic centimeters inordinary higher apes, and nearly 1,500 cubic centimeters in the average modern man. The very recently extinct very low-type aborigines of Tasmania had a skull capacity of 1,199 cubic centimeters.

In 1907 the lower jaw of an anthropoid or manlike ape set with rather human teeth was found associated with very crude stone implements seventy-five feet below the surface in river-deposited sand in Germany. It is of either early or middle Glacial time and quite certainly represents a lower order creature than the oldest Paleolithic man as described below.

Many bones and implements of Paleolithic man (seeabove table) have been found mainly in river gravels and caves. The relative ages of Paleolithic human bones and implements are best determined by the associated fossil animals. Thus the most ancient truly human fossils are found directly associated with bones of very old types of elephants, rhinoceroses, and hippopotamuses which are definitely known to have lived during middle or early middle Glacial (Quaternary) time corresponding to early Paleolithic time. A very conservative estimate would make the age of such very old human remains at least 150,000 to 250,000 years because the Ice Age was at least 500,000 years long. In a later human stage there are many associations with extinct animals like an older type of mammoth, cave bear, cave hyena, and others of later Glacial time estimated at 50,000 to 150,000 years ago. Last of all was the latest Paleolithic stage corresponding to the close of the Ice Age, the human remains of which are found associated with reindeer and the latest mammoths which roamed in great numbers across Europe. This was probably not more than 30,000 to 50,000 years ago.

Paleolithic man is so called because he fashioned stone weapons and implements. The structure of skull and skeleton shows him to have been a low-type savage, something over five feet high on the average, with a forward stooping carriage. The average Paleolithic brain was not greatly inferior in size to that of modern civilized man, but it was not so highly organized and occupied a thick skull with much lower forehead and heavy brow ridges. The bushmen of Australia and the recently extinct Tasmanians are the nearest modern resemblances. Many fine specimens of Paleolithic man have been found, especially in cave deposits. That he was an expert hunter is proved by the great accumulation of bones of now extinct animals found in and about his haunts or camps, bones representing at least 100,000 horses having been found around a single camp site!

Fig. 68.—Comparison of skulls: a, Paleolithic (Neanderthal) man; b, modern man. (After Woodward, British Museum.)

Only two among the many known Paleolithic man localities will be briefly described. In the Perigord district of southwestern France a number of caves contain human relics ranging in age from early to late Paleolithic. Of special interest among theserelics are fishhooks made of bone, and crude sketches of animals such as the mammoth and reindeer now extinct in that region. The Aurignac cave, also in France, was no doubt a family or tribal burial place. Seventeen Paleolithic human skeletons, associated with bones of extinct animals and crude art works, were found in the cave. Near the entrance there were ashes and charcoal mixed with burned and split bones of extinct animals. Certain of the caves occupied by late Paleolithic man have their walls decorated with sketches and even colored pictures. These are, therefore, the oldest known art galleries. An excellent example is the cave at Altamira in northern Spain. “As we glance at the pictures one of the first things to impress us is the excellence of the drawing, the proportions and postures being unusually good.... The next observation may be that, in spite of this perfection of technique, there is no perspective composition—that is, no attempt to combine or group the figures.... It is also clear that the work of many different artists is represented, covering a considerable period of time. The walls show traces of many other paintings that were erased to make way for new work.” (Wissler.)

Fig. 69.—Sketch of a painting by Paleolithic man found in a cave in west-central France. Various animals, including the extinct mammoth elephant, are represented. (Courtesy of American Museum of Natural History.)

The Neolithic, or “recent stone” age was a gradual development from the late Paleolithic, and man was then more highly developed and more similar in structure to modern man. His stone implements were more perfectly made, and often more or less polished and ground at the edges. “The remains of Neolithic man are found, much as are those of the North American Indians, upon or near the surface, in burial mounds, in shell heaps (the refuse heaps of their settlements), in peat bogs, caves, recent flood-plain deposits, and in beds of lakes near shore where they sometimes built their dwellings upon piles.... Neolithic man in Europe had learned to make pottery, to spin and weave linen, to hew timber, and build boats, and to grow wheat and barley. The dog, horse, ox, sheep, goat, and hog had been domesticated.” (Norton.)

“Man is linked to the past through the system of life, of which he is the last, the completing creation. But, unlike other species of that closing system of the past, he, through his spiritual nature, is more intimately connected with the opening future.” (J. D. Dana.)

CHAPTER XX

MINERALOGY

WE are more or less familiar with the division of all materials of nature into the animal, vegetable, and mineral kingdoms. With slight exceptions minerals are the materials which make up the known part of the earth. In a very real sense, then, mineralogy is the most fundamental of the various branches of the great science of geology because the events of earth history, as interpreted by the geologist, are recorded in the mineral matter (including most rocks) of the earth. When we examine the rocky material or mineral matter of the earth in any region we find that it consists of various kinds of substances each of which may be recognized by certain characteristics. Each definite substance (barring those of organic origin) is called a mineral. Or, more specifically, a mineral is a natural, inorganic, homogeneous substance of definite chemical composition. According to this definition a mineral must be found ready made in nature, must not be a product of life, must be of the same nature throughout, and its composition must be so definite that it can be expressed by a chemical formula. All artificial substances, such as laboratory and furnace products, are excluded from the category of minerals. Coal is not a mineral because it is both organic and of indefinite composition. A few examples of very common substances which perfectlysatisfy the definition of a mineral are quartz, feldspar, mica, calcite, and magnetite. Only two substances—water and mercury—are ordinarily liquid minerals. There are nearly a thousand distinct mineral species, and to them and their varieties several thousand names have been applied.

It is a surprising fact that of the eighty or more chemical elements, that is substances which cannot be subdivided into simpler ones, only eight make up more than 98 per cent of the weight of the crust of the earth, though, with one very slight exception, none of the eight exist as such in mineral form. The eight elements are oxygen (nearly 50 per cent), silicon (over 25 per cent), aluminum (over 7 per cent), iron (over 5 per cent), calcium (or “lime”), magnesium (or “magnesia”), sodium (or “soda”), and potassium (or “potash”).

Certain rock formations are made up essentially of but one mineral in the form of numerous grains as, for example, limestone, which consists of calcite (carbonate of lime). Most of the ordinary rocks are, however, made up of two or more minerals mechanically bound together. Thus, in a specimen of granite on the author’s desk several distinct mineral substances are distinguishable by the naked eye. These mineral grains are from one to five millimeters across. Most common among them are hard, clear, glassy grains called quartz; nearly white, hard grains, with smooth faces, called feldspar; small, silvery white plates, easily separable into very thin flakes, called mica; and small, hard, black grains, called magnetite. It is the business of the mineralogist to learn the characters of each mineral, how they may be distinguished from each other, how they may be classified, how they arefound in nature, and what economic value they may have. It is an important part of the business of the geologist to learn what individual minerals combine to form the many kinds of rocks, how such rocks originate, what changes they have undergone, and what geological history they record. It is thus clear that the great science of geology is much broader in its scope than mineralogy.

One of the most remarkable facts about minerals is that most of them by far have a crystalline structure, that is they are built up of tiny particles known as molecules. Such crystalline minerals are often more or less regular solid forms bounded by plane faces and sharp angles, such forms being known as “crystals.” How do crystals develop such regularity of form? Any solid is considered to be made up of many very tiny (submicroscopic) molecules held together by an attractive force called cohesion. In liquids the molecules may more or less freely roll over each other, thus altering the shape of the mass without disrupting it. In gases the molecules are considered to be relatively long distances apart and moving rapidly. During the process of change of a substance from the condition of a liquid or gas to that of a solid, due to lowering of temperature or evaporation, the cohesive force pulls the particles (molecules) together into a rigid mass. Under favorable conditions such a solid has a regular polyhedral form. "This results from the fact that the particles or molecules of the substance which, while it was liquid or gaseous, rolled about on one another, have been in some way arranged, grouped and built up. To illustrate this, suppose a quantity of small shot to be poured into a glass: the shot will represent the molecules of a substancein the liquid state, as for example a solution of alum. If, now, we suppose these same shot to be coated with varnish or glue so that they will adhere to each other, and imagine them grouped as shown in Figure 70a, they will represent the arrangement of the molecules of the alum after it has become solid or crystallized. This arranging, grouping, and piling up of molecules is called crystallization, and the solid formed in this way is called a crystal. Figures 70b and 70c show the shot arranged to reproduce two common forms of crystals (e.g., fluorite and calcite)." (Whitlock.)

Fig. 70.—Piles of shot arranged to give some idea of the manner in which molecules are bound together in various crystal forms. (After Whitlock, New York Museum.)

A combination of certain facts regarding crystals furnish all but absolute proof of some sort of regularity of arrangement of particles within them. Among such facts are the following: (1) the wonderful regularity of arrangement of faces upon crystals is practically impossible to account for except as the outward manifestation of regularity of structure or systematic network arrangement of the interior; (2) most crystals split or cleave more or less perfectly in one or more directions presumably in accordance with certain layered structure of the constituent particles; (3) all of the many known forms of crystals can be accurately grouped in regardto their effects upon the passage of light (especially polarized light) through them, each kind or type of network structure presumably producing a different effect upon light; and (4) X-ray photographs have proved that particles, or at least groups of particles, are very systematically arranged within crystals.

It will be instructive for us to make a comparison between the growth of crystals and organisms. Both really grow, but each species of organism is rather definitely limited in size while there is no known limit to the size which may be attained by a crystal so long as material is supplied to it under proper conditions. As a matter of fact crystals vary in size from microscopic to several feet in length, those less than an inch in length being most abundant by far. Organisms mostly grow from within, while crystals grow from material externally added. It is an astonishing fact that in crystals as well as organisms growth takes most rapidly on a wound or broken place. Thus if a crystal is removed from the solution in which it is growing and put back after a corner has been broken off, the fractured surface will build up more rapidly than the rest. Finally, crystals are not necessarily limited in age like organisms. Under certain natural conditions, as, for example, weathering, crystals may decay or be broken up; but where they are protected as constituent parts of rock formations well below the earth’s surface they may remain unchanged for indefinite millions of years. Thus in a ledge of the most ancient known or Archeozoic rock only recently laid bare by erosion one may see crystals which are precisely as they were when they crystallized many millions of years ago.

One of the most remarkable properties of a crystal is its symmetry, by which is meant the greater or less degree of regularity in the arrangement of its faces, edges, and vertices. A given substance may, according to circumstances, crystallize in a variety of forms or combinations of forms, but, with very few exceptions, all crystals of a given substance exhibit the same kind or grade of symmetry. There are three kinds of crystal symmetry, namely, in respect to a plane, a line or axis, and a point or center. A plane of symmetry divides a crystal into halves in such a way that for every point on one side of the plane there is a corresponding point directly opposite on the other side. Crystals may be cut into halves along various surfaces which are not symmetry planes. An axis of symmetry is a line about which a complete rotation (or in a few cases rotation combined with reflection) brings the crystal into the same relative position two, three, four or six times, these being called two, three, four, and sixfold axes of symmetry—no others being possible. A crystal has a center of symmetry when any line passing through it encounters corresponding points at equal distances from it on opposite sides. There are just 32 classes or combinations of the symmetry elements among crystals and just 232 definite crystal forms. Not only is it demonstrable that no more can exist, but actual experience with crystals of hundreds of species of minerals has never revealed any more. Obviously, then, symmetry furnishes us with a very scientific basis of classification of crystals, all of the 232 crystal forms constituting the 32 symmetry classes being in turn referable to seven fundamental crystal systems. To bring out the relations of the faces of a crystaland further aid in classification, prominent, straight lines or directions passing through the center of a crystal are chosen as crystallographic axes. Such axes may or may not coincide with symmetry axes. Basing our definitions upon both symmetry axes and crystallographic axes, the seven systems are as follows:

1. Isometric. There must be at least four threefold axes of symmetry, while the highest grade symmetry class of the five in the system includes three fourfold, four threefold, and six twofold axes of symmetry; nine planes of symmetry; and a center of symmetry. There are three interchangeable crystallographic axes at right angles to each other.

Fig. 71.—Figures showing, a, crystal axes of Isometric system; b, points of emergence of the nine axes of symmetry in a cube of the Isometric system; c, nine planes of symmetry in a cubic crystal. (After Whitlock, New York State Museum.)

2. Tetragonal. There must be one and only one fourfold symmetry axis, while the highest of its seven symmetry classes contains also four twofold axes of symmetry; five planes; and a center. Characterized by three crystallographic axes at right angles to each other, only two of them interchangeable.

3. Trigonal. Characterized by one and only one threefold symmetry axis, the highest of the fiveclasses having also three twofold axes; four planes; and a center. Crystallographic axes as for hexagonal.

4. Hexagonal. One and only one sixfold axis of symmetry must be present, but the highest of the seven classes also has six twofold axes; seven planes; and a center. Characterized by four crystallographic axes, one vertical and three interchangeable horizontal axes making angles of 60 degrees with each other.

5. Orthorhombic. There must be no axis of symmetry higher than a twofold and three prominent directions (i.e., parallel to important faces) at right angles to each other, the highest grade of the three classes having three twofold axes; three planes; and a center. There are three noninterchangeable crystallographic axes at right angles.

6. Monoclinic. There is no axis of symmetry higher than a twofold and only two prominent directions at right angles to each other, the highest of the three classes having one twofold axis; one plane; and a center. There are three noninterchangeable crystallographic axes, only two of which are at right angles.

7. Triclinic. There is no axis of symmetry of any kind, and there are no prominent directions at right angles. One of the two classes has a center of symmetry only, and the other no symmetry at all. Characterized by three noninterchangeable crystallographic axes, none at right angles.

A fact which should be strongly emphasized is that crystals only, of all the objects of nature, can be definitely referred to the above seven systemscomprising the 32 classes of symmetry, and 232 crystal forms. Since there are about 1,000 mineral species and only 232 fundamental forms, it necessarily follows that two or more species may crystallize in the same form within a class, so that it is not always possible to tell the species of mineral merely by its crystal form. It is, however, a remarkable fact that, where two or more substances crystallize in the same class (i.e., show the same grade of symmetry) each substance almost invariably exhibits “crystal habit” which is a pronounced tendency to crystallize in certain relatively few forms or combinations of forms out of the many possibilities. It is clear, then, that grade of symmetry combined with “habit” are of great practical value in determining crystallized minerals, because, on the basis of symmetry, a crystal is referred to a certain definite symmetry class in which only a limited number of substances crystallize, and then, by its characteristic “habit,” the particular substance can be told.

Fig. 72.—Figures illustrating three crystal forms with exactly the same symmetry elements; a and b are separate forms, and c is a combination of the two. The mineral “garnet” nearly always crystallizes in one of these forms.

From the above discussion it should not be presumed that crystals always develop with perfect geometric symmetry. As a matter of fact such isseldom the case because, due to variations of conditions or interference of surrounding crystals in liquids (ordinary or molten), a crystal usually grows more rapidly (by building out faces) in certain directions than in others. Under such conditions actual crystals are said to become distorted because they are not geometrically perfect.

Whether geometrically perfect or not, all crystals respond to the law of constancy of interfacial angles which means that on all crystals of the same substances the angles between similar (corresponding) faces are always equal. This is one of the most fundamental and remarkable laws of minerals. That it must be true follows from the fact that the crystal faces merely outwardly express in definite form the definite internal structure or arrangement of particles which have built up the crystal. In other words, the real structural symmetry of a crystal never varies no matter how much its geometric symmetry may vary. The practical application of the law of constancy of interfacial angles lies in the fact that in many cases a mineral may actually be identified merely by measuring the interfacial angles of its crystal form.

The relative lengths of the crystallographic axes is a very important feature of all crystals except those of the isometric system in which the axes are always of equal length so that the ratio is 1:1:1. In all the other systems, however, at least one axis differs in length from the others and, since the amount of difference is absolutely characteristic of each substance, the axial ratio of a crystal, when carefully determined by measurement of the angles between the different faces, affords a never-failing method of determining the mineral forall systems except the isometric. By way of illustration, the tetragonal crystal of the mineral zircon, with only one axis different in length, shows the very definite axial ratio 1:1:0.64, while the orthorhombic crystal of sulphur, with all three axes of different lengths, has an axial ratio 0.813:1:1.903. These ratios of course always hold true no matter what the size or particular outward form of the crystal.

As might be expected from the above discussion of the remarkable structure of crystals, experience has proved that the relative lengths of all intercepts (or distances from the center) of all faces upon any crystal can be expressed by whole numbers, definite fractions, or infinity. It necessarily follows that the ratios between the intercepts of the faces of any face on a crystal to those of any other face on the same crystal may always be expressed by rational numbers, and this is known as the law of definite mathematical ratio. It is a remarkable fact that very small whole numbers or fractions, or infinity or zero, will always express the intercepts of any crystal face.

Thus far our discussion has centered about crystals as individuals, but, in most cases by far, they form groups or aggregates. Most commonly crystal grouping is very irregular, but by no means rare is parallel grouping where whole crystals, or more usually parts of crystals, have all corresponding parts exactly parallel. But most remarkable of all are the twin crystals in which two or more crystals intergrown or in contact have all corresponding parts in exactly reverse order. The conditioning circumstances under which twin crystals develop are unknown.

In the light of the facts and principles above explained, the reader will more than likely agree with the author that crystals rank very high among nature’s most wonderful objects. But there are still other characteristic features of crystals naturally resulting from their marvelous structure. Some of these will now be briefly referred to.

Fig. 73.—Figures illustrating twin crystals: a, gypsum (Monoclinic system); b, fluorite (Isometric system); c, cassiterite (Tetragonal system). (After New York State Museum Bulletin.)

Many crystals and crystalline substances exhibit the important property known as cleavage which is the marked tendency to break easily in certain directions yielding more or less smooth plane surfaces. As would be expected, a cleavage surface is always parallel to an actual, or at least a possible, crystal face, and it takes place along the surfaces of weaker molecular cohesion. The degree of cleavage varies from almost perfect, as in mica, to very poor or none at all, as in quartz. The number of cleavage directions exhibited by common minerals is illustrated as follows: mica, one; feldspar, two; calcite, three; and fluorite, four.

It is a striking fact that when a crystal or cleavage piece is placed in a solvent, the action proceeds with different velocities in crystallographically different directions and little pits or cavities, calledetching figures, are developed on some or all of the faces. Since the symmetry of these etching figures and their arrangement upon the faces are directly related to, and natural effects of the crystal symmetry, the figures often furnish an important method of placing a doubtful crystal or even merely a cleavage fragment in its proper symmetry class.

Another marvelous property of crystals and crystalline substances is their effect upon light. Since the study of the passage of light through crystals has really become a large separate branch of mineralogical study, we can no more than state a few fundamental facts and principles in the short space at our disposal. Light is caused by vibrations of the so-called “ether,” and always travels in straight lines. The vibration directions are at right angles to the direction of transmission of the light. When a ray of light enters a crystal or crystalline mineral representing any crystal system except the isometric it is doubly refracted (i.e., broken into two rays), each of the two rays is polarized (i.e., made to vibrate in a single plane only), and one ray vibrates almost at right angles to the other. Double refraction is strikingly shown by placing a piece of clear calcite (Iceland spar) over a dot on paper when two dots instead of one are visible. The amount of double refraction varies with the substance, and in some degree according to the direction of passage of light through a crystal. Isometric crystals only are singly refracting and hence a ray of light is not affected in passing through them. Crystals of all the other six systems doubly refract and polarize light and in three systems—tetragonal, hexagonal, and trigonal—one direction (coincident with the main axis of symmetry) produces single refractiononly, while in the remaining three systems—orthorhombic, monoclinic, and triclinic—there are always two directions of single refraction whose positions vary with the substance. Many crystals outside the isometric system also exhibit a remarkable tendency to absorb light differently in different crystallographic directions, thus producing two or three color tints, which vary according to the substance. After gaining a practical knowledge of the above and many other optical properties of crystals, it is possible by the aid of a specially constructed (polarizing) microscope, to recognize (with few exceptions) each one of the many mineral species. This method is of great value in determining the various minerals which are aggregated in the form of a rock, in which case a very thin slice of the rock is studied with the microscope.

An important criterion for the recognition of minerals is hardness, by which is meant the resistance of a smooth surface to abrasion or scratching. The generally adopted scale of hardness follows:

1.—Soft, greasy feel, and easily scratched by the finger nail (e.g., talc).

2.—Just scratched by the finger nail (e.g., gypsum).

3.—Just scratched by a copper coin (e.g., calcite).

4.—Easily cut by a knife, but does not cut glass (e.g., fluorite).

5.—Just scratches soft glass, and is cut by a knife (e.g., apatite).

6.—Harder than steel, and scratches glass easily (e.g., orthoclase).

7, 8, 9, and 10.—Harder than any ordinary substance and represented in order by quartz, topaz, corundum, and diamond.

Plate 17.—Skeleton of the Great Two-Legged, Carnivorous Dinosaur Reptile, Called “Tyrannosaurus,” Which Lived During Cretaceous Time.(Courtesy American Museum of Natural History.)Small Picture.—Restoration of the Earliest Known Bird of Which Several Nearly Perfect Skeletons Have Been Found.This feathered creature with reptilian characteristics lived at least 5,000,000 years ago. It had a long vertebrated tail, claws on the ends of the wings, and teeth. (By E. W. Berry.)

Plate 17.—Skeleton of the Great Two-Legged, Carnivorous Dinosaur Reptile, Called “Tyrannosaurus,” Which Lived During Cretaceous Time.(Courtesy American Museum of Natural History.)

Small Picture.—Restoration of the Earliest Known Bird of Which Several Nearly Perfect Skeletons Have Been Found.This feathered creature with reptilian characteristics lived at least 5,000,000 years ago. It had a long vertebrated tail, claws on the ends of the wings, and teeth. (By E. W. Berry.)

Plate 18.—(a)Skeleton of the Largest Known Creature That Ever Flew.It was a flying reptile with spread wings of nearly twenty-five feet, and lived during the Cretaceous period several million years ago. (Courtesy of the American Museum of Natural History.)

Plate 18.—(b)Skeleton of a Remarkable Swimming Reptile of the Mesozoic Era.Length about twelve feet. Parts of skeletons of unborn young are seen. (Courtesy of the American Museum of Natural History.)

Minerals also show a great variety of colors. Many of them like quartz and calcite are colorless or white, others like galena (steel-gray) and pyrite (brass-yellow) show inherently characteristic colors, while still others like amethyst (purple) and sapphire (blue) are colored by impurities.

There is also a great range in relative weights or density of minerals, commonly called the specific gravity, which range from less than one for ice to 21.5 for platinum, and even somewhat higher. The average specific gravity of all minerals of the earth is about 2.6.

In the light of the above discussion of the general properties of minerals, we shall now proceed to name and briefly describe some of the minerals which are either very common, or of special interest, or of special economic importance. Only those features are listed by which the mineral species may be recognized at sight, or by the aid of very simple nonchemical tests.

Fig. 74.—Drawings showing forms of crystals of common minerals: a and b, garnet (Isometric); c and d, feldspars (Monoclinic); e, f, and g, quartz (Trigonal); h, i, and j, calcite (Hexagonal); k, augite (Monoclinic); l. hornblende (Monoclinic); m, pyrite (Isometric).

Amphibole.A number of species closely related in composition, crystal form, and properties are here included. They are silicates of lime and magnesia usually with aluminum and iron. Most common by far are those which crystallize in the monoclinic system with prismatic faces and two good prismatic cleavages meeting at about 24 degrees. Color, commonly brown to black, but sometimes green or white. Hardness varies from 5 to 6, and specific gravity from 3 to 3.4.Hornblende, the most common species, is a dark colored silicate of lime, magnesia, aluminum, and iron. It is one of the few most common of all mineral species, especially in igneous and metamorphic rocks.Tremoliteis a white to light gray silicate of lime and magnesia found especially in metamorphic limestones.Actinoliteis a green silicate of lime, magnesia, and ironespecially common in certain metamorphic rocks. One kind of jade is an amphibole similar to tremolite and actinolite in composition, while the other kind is a pyroxene (seebelow).Jadeis and has been highly prized in the east (especially in China)where it has been carved into many objects of exceptional variety and beauty. Jade is probably the toughest (not hardest) of all minerals because of its wonderful microscopically fibrous structure. In color it is white, gray, and green.

Apatite.Crystallizes in the hexagonal system with a six-sided prism usually capped at each end by a six-sided pyramid (seeFigure 75g). Composition, a phosphate of lime. Color variable, but mostly white, green, or brown. Hardness of 5, or just enough to scratch soft glass. Specific gravity, 3.2. No good cleavage. Tiny crystals are widely disseminated through many common rocks—igneous, metamorphic, and sedimentary. In certain metamorphic limestones excellent crystals a foot or more in length have been found. Apatite, mostly in uncrystallized form, is the source of most of our phosphate fertilizers.

Azurite.An azure-blue hydrous carbonate of copper which crystallizes commonly in small monoclinic crystals. Hardness, nearly 4, and specific gravity, nearly 4. Commonly occurs in veins deposited by underground water. One of the great ores of copper, especially in Arizona, Chile, and Australia.

Barite.A sulphate of barium crystallizing in orthorhombic prisms usually of tabular habit. White to light color shades. Hardness, 3.5; specific gravity, 4.5, which is notably higher than the average of light-colored minerals. Three good cleavages parallel to principal crystal faces. A common and widely distributed mineral, especially in many vein deposits associated with certain ores. Used in ground form to give weight to certain kinds of paper and cloth, and a barium compound used for refining sugar is made from it.

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