LESSON XIV.

LESSON XIV.

FOOTPRINTS IN THE SAND.

“So where some trivial creature played of oldThe warm soft clay received the tiny dint;We cleave the rock’s deep bosom, and beholdDeep in its core, the immemorial print.Men marvel such frail record should outlineThe vanished forests and the trees o’erhurled.”

“So where some trivial creature played of oldThe warm soft clay received the tiny dint;We cleave the rock’s deep bosom, and beholdDeep in its core, the immemorial print.Men marvel such frail record should outlineThe vanished forests and the trees o’erhurled.”

“So where some trivial creature played of oldThe warm soft clay received the tiny dint;We cleave the rock’s deep bosom, and beholdDeep in its core, the immemorial print.Men marvel such frail record should outlineThe vanished forests and the trees o’erhurled.”

“So where some trivial creature played of old

The warm soft clay received the tiny dint;

We cleave the rock’s deep bosom, and behold

Deep in its core, the immemorial print.

Men marvel such frail record should outline

The vanished forests and the trees o’erhurled.”

A teacher and his class sat one day upon some tree-trunks which had fallen across a bed of bare, slightly-sloping rocks. The teacher, with his head between his hands, looked steadfastly down. “What are you thinking of?” demanded a pupil.

“I am reading,” said the teacher.

“And what do you read? Is the rock your book?”

“I read that this stratum of rock was once a beach of sand. It was just such sand as a child likes to play in; just such sand as it is fabled that the Sibyl took in her hands when she asked long life of Apollo.”

“And how did it become rock?”

“You must know that all new rocks are made of the wear and tear of older rocks, reconstituted and relaid. This sandstone was once sand, and the sand was chiefly tiny quartz crystals, the residue left after the softer mineral portions of certain ground-up older rocks had been carried away. These quartz crystals could not be melted, nor held in solution by water; being heavy they could not be held in suspension by water. So the water which was carrying about the rubbishof the older rocks, dropped the sand, and it formed long beaches, such as we see to-day. Observe that sand seems to be more plentiful than any other substance in the earth’s crust. Most soil is largely composed of it. If it were not for the sand in the soil to keep its particles loose and allow the percolation of water, and the growth of roots, the soil would soon become packed so heavily that almost nothing could grow. The sea-shore, the river-beds, the great Sahara, are sand, sand, sand. As sand is the most plentiful loose material in the earth now, so no doubt it has always been, and we find that sandstone, namely, rock made of sand, is the most abundant in the structure of the globe. The grains of quartz themselves are naturally gray or white, but they are covered with a film or crust which takes different hues from coloring matter mixed with it. In iron regions the sandstone is red, and in anthracite coal regions the sandstone is dark gray or nearly black. Other sandstones are brown, yellow, or purple, according as they are mixed with other coloring matter.

“But you asked me how the loose sand became rock. Understand that the most important factor in the process was an indefinitely long time, while the agents required were very simple—great pressure and the presence of water. The water carrying with it silica, or some kindred material, formed a cement about each grain, and thus aided by pressure and a certain amount of heat—for in this world nothing is done without heat—the individual grains were finally consolidated into massive rock.”

“What else have you read?” asked other pupils, asthey too began earnestly to contemplate the ledge of sandstone.

“That was only the preface, or first chapter. I remember that the grains of sand, composed largely of silica or tiny quartz crystals, were heaped up here by water, which washed from the crystals and then carried away, little particles of lighter matter, as mud. When I look at these long, gently sloping ledges of sandstone, I read that they were once a beach. The ravine and the hills, the cornfields and the forests are now here, but once—the sea. Warm salt waters ebbed and flowed here, and shells were rolled up on this beach and were carried out again by the tides.”

“Can you read any thing further?”

“Yes: One day, long ago,—was it at the close of the Carboniferous or the beginning of the Permian age?—there was a rain-storm here. It came up with a strong wind, from the south; the drops fell heavily, and were very large. I think it must have been what we call a ‘black squall.’ It soon passed, and the sun shone out hot.”

“Oh, oh! now you are guessing at what might have been!”

“Not at all. I am reading what really was. Are not these rocks my books? Look now at this ledge. Here is a thin layer of mud-rock, as well as of sandstone. Here was some marl-mud laid down. I think that here must have been an inlet. Observe here on this exposed surface, these pits or dents. You can put your finger tip in them. See how they slant from the south, and how deeply they are driven into the sand. Suppose now that we stood by a lowlevel beach of sand and marl-mud, and a sharp shower, driven by a strong wind, came up from the south, and the big rain-drops pelted violently on the sand and mud. Would they not leave marks identical with these? So we know that here were big rain-drops from a southerly gust. We know that it was soon over; for if rain had fallen long, all the surface of the sand would have been washed and saturated, and prints of individual drops would have been obliterated. We know that the shower was followed by a hot sun, and as the sun was very hot, we infer that the time was somewhere in the middle of the day, between ten and four, the time when the sun rays are most powerful. We know that the sun was hot, because these rain-marks dried quickly, before they had time to lose the sharpness of their imprint. It is also evident that such delicate markings must have been quickly covered up, else the next heavy mist or drizzle of rain, or creeping tide, would have smoothed them out. Therefore, I make a guess that when the marks had dried hard, being sun-baked as some bricks are, a strong breeze came up, sweeping sand before it, and so buried the prints under sufficient sand to protect them; or, possibly, a fresh layer of ooze, or mud and sand mixed, came down the estuary and rolled gently over these rain-prints and cased them safely.”

“Who would have thought of reading all that in sand!”

“Do we not daily read footprints in sand in this fashion? As we came here we crossed the road lying deep in summer dust. There in the dust were the prints of a three-toed foot.We recognized the size and shape; it was the track of a hen’s foot; a large fat hen we judged, because the prints were large and deeply indented. Around her tracks were the small delicate marks of the feet of young chicks. There were many of them; evidently the hen had a large brood with her. Here she had scratched in the dust where some grain or meal had been scattered: here was a deep hollow where the dust was flirted about on all sides; the hen had been taking a dust-bath. Here were some much larger marks of three-toed feet, their distance apart showing a longer stride: we felt quite certain that a big rooster had marched that way. We knew about the hen, the chicks, the cock, the feeding, the wallowing, as well as if we had watched the birds themselves and their performances. But we had only seen footprints in the sand. Has any one of us wondered at Robinson Crusoe’s conclusions when on the beach he found the print of a naked human foot?”

The pupils admitted the justice of this reasoning. “What else have you read?” they questioned.

“Shortly after the shower an animal passed this way; a two-legged animal with three-toed feet, and a stride six feet in length. Such a creature would make an ostrich seem small. Whether it was a bird or a lizard I cannot tell, at all events it has neither kindred nor descendants in the modern era. If a bird, it was of that antique type which had teeth and no feathers and wore a lizard’s tail. It was probably a wader of some kind, and ate mollusks and fish. I say that it passed this way about the time of the shower, because I find its footprints near the rain-marks, and on the same level ofrock, and because they seem to have been covered up about the same time, and in the same manner.”

“Do you suppose if we broke up some of this rock we should find remains of the living creatures that moved over it?”

“No; sandstone is not rich in fossils. Perhaps the oxide of iron that frequently accompanies the sand has destroyed plant and animal organisms, or it may be that when the heavy quartz grains dropped out of the water, the plant and animal remains were carried off by tides or currents. Still, in some parts of the world fossils are found in sandstones.”

“Do all the strata of sandstone tell this same story?”

“Each varies the story somewhat to suit its own circumstances. Some tell us which way the tides flowed and the rivers ran that brought them to their resting places long ago.”

“Are all sandstones alike, except in color?”

“No; some sandstone, especially that lying near the surface of the ground is soft, and the grains are loosely held together, and are large and distinct. Other sandstone has been pressed together into a mass nearly as hard as marble or granite. Flagstone is sandstone laid down in thin beds, and capable of being readily split into flags or slabs. Freestone is a soft sandstone which is capable of being readily cut in any direction; buhrstone is a particularly hard, rough sandstone, of which grindstones for mills are made. Roll yonder great, rough stone here. What do you think of it?”

“It looks like plum-duff,” said one of the pupils, “only the plums are rather large and numerous, and the dough is very hard and scanty.”

“It is conglomerate,” said the teacher, “and conglomeratediffers from sandstone in the size of its grains. Where sandstone has tiny quartz crystals for grains, conglomerate has pebbles, fastened together with mud hardened like cement. Coarse sandstone closely resembles fine conglomerate. Another kind of sandstone is called greywacke. Greywacke often exhibits the ripples and sun-cracks of the ancient beach. In greywacke the grains are rounded, or nearly rounded, are uncolored, and are cemented together by a very hard paste of chalk or silica. If you look at a piece of greywacke you might think it a portion of igneous or fire-made rock, it is so fine and hard. You must remember that rocks which have been—let us say, well cooked, by the earth’s internal fires, are very hard and compact; they have been fused together and then cooled, and the process has made them very strongly coherent. But sandstone in all its varieties is a water-made, not a fire-made rock. These water-made rocks are called fragmental, while fire-made rocks are termed igneous. Whatever heat has been applied to sandstone has served only to melt and harden the fine cement, but has not been sufficient to fuse the grains. The term fragmental, as applied to rocks, means that the parts are easily divisible, and are largely individual and fragmentary. Clay rocks are fragmental, and so are limestone rocks. There are also a few of the fragmental rocks which have been very greatly heated, and indeed were once flung out of volcanoes in a red-hot or melted state. But as they are in a loose condition much like cinder and ashes, they are classed among the fragmental rocks. Among these are tufa, breccia, volcanic conglomerate, and two or three others.”

“It needs much learning to read so much from a few footprints in sand,” said one of the pupils, as they rose to go homeward.

“Say, rather,” said the teacher, “that by close observation and by comparison of observed facts, we may gather more or less valuable information. Pupils are too ready to depend for information rather upon memorizing what is in books than upon the cultivation of the faculties of observation. Now, the books are very valuable as giving us the knowledge amassed by many minds, and also they help us to reason judiciously upon what we see. But the information gathered from books is much more easily forgotten than that which comes to us from observation, and we must, in the study of natural science, yoke the book to practical observation, and closet study to out-of-doors study.”

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