Читать книгу Walks and Talks in the Geological Field - Alexander Winchell - Страница 17

It is not entirely satisfactory to roam over the fields, with bowlders lying on the right and left, but without any knowledge of their names or natures. True, we shall experience much satisfaction in feeling that we know something of their origin and their history. We may walk up to the side of one of these ancient and way-worn travelers and say: “Old Hard Head, when did you arrive in this country, and where did you emigrate from?” Old Hard Head will lie sullenly and answer never a word. But he is written all over with inscriptions which we can already begin to decipher. So we look on the rounded and weather-beaten form, and say to ourselves: “This immigrant rock came from a northern country. He left his mother-rock, and most of his kindred, in the woods of northern Maine, or New Hampshire, or on the shore of Lake Superior. A large number of his kindred came with him. He rode part of the way on the back of a glacier. By and by he fell off, or got into a hole; and after that he had a severe squeezing. He got crushed and rubbed and rolled and pushed for some thousands of years. But every year he made some progress. By and by there was a great change of weather. The ice-carriage melted away from him, and fine weather returned, and lo! he found himself, one spring, in this field. That was long enough before Adam and Eve set up business in gardening. But here old Hard Head has been lying ever since. And now, we are the very first persons who ever stopped to pay him a moment’s attention, and make his acquaintance.”

If old Hard Head thinks, he is revolving some handsome compliments on our intelligence. Whatever old Hard Head may think, we are sure the ability to learn something of the method of the world was given us to be exercised. If we go stupidly through the world, without exercising that ability, we do no better than an ox. But if we seek to gain an insight into the method and history of the world, we honor the Author of the world; we read His thoughts. Knowing some of His thoughts, we come into more intimate relations with him. The study of science is a virtue. Attention to geology is a human duty.

To complete our introduction to old Hard Head we must know his name. To call him old Hard Head is like calling a man “Old Russian” or “Old Englishman.” He has, besides, his personal name. Now, there is a way of finding out the particular name of each rock. Like a dog with his name on his collar, each mute rock displays a name written on its exterior. Let us look into this subject a few minutes.

Do you see that nearly all these bowlders appear to be mixtures of different colors and kinds of rocks? See one rock with round pebbles—white, red, black—imbedded in a mass of smaller grains. See another rock, less coarse, with grains white, pink, and black. See still another with grains all nearly alike. See one rock nearly a uniform white; another, bluish; another, reddish; another, nearly black. See one rock with numerous black shining scales; another, with smoky scales; another, with silvery scales. Now, all these differently colored constituents of the rocks are so many different minerals. Rocks are composed of minerals. Some rocks have two minerals; some, three; some, four; and some, only one. The particular name of a rock depends on the minerals in it. As soon as we know the minerals, we can call the name of the rock. Now, sit down and take a lesson in minerals.

Do you see this white flint rock, composed throughout of one kind of mineral? That mineral is Quartz. It is the hardest of all the common minerals. Try to scratch it. You see the point of steel makes no impression on it. But it leaves a black mark. The Quartz wears away the steel. When one of these bowlders is thus composed entirely of Quartz, its name is Quartzite. There are many Quartzites, as there are many Smiths and Joneses. Let us learn the other part of the name. Look at these uniformly colored quartzites—white and gray. You see one is composed of distinct grains; this is a Granular Quartzite. One has the grains almost completely melted together or confluent; this is a Vitreous Quartzite. One contains pebbles; this is a Conglomeritic Quartzite, or simply a Conglomerate. Another has some of its pebbles red; this is a Jaspery Conglomerate. You will find quartzites exceedingly abundant; and you will find grains of quartz in many other rocks than quartzites. In fact, quartz is most abundant of all minerals.

Conglomerates always excite curiosity—especially if the pebbles are of different bright colors. Two large masses of conglomerates of different sorts lie on the campus of the University of Michigan—souvenirs of two graduating classes. One is a jaspery conglomerate from the north shore of Lake Huron. It weighs six tons. The plum pudding, as big as the State House dome, demolished by the giant’s wife and her screaming boys, refers to a conglomerate.

“They flung it over to Roxbury hills, They flung it over the plain; And all over Milton and Dorchester, too, Great lumps of pudding the giants threw; They tumbled as thick as rain.”

Well, here is a rock with shining scale-like mineral fragments. Pick up a scale with your knife-blade. Do you see it split into laminæ or leaves of indefinite thinness? “Yes,” you say; “this is the same thing as is used in the doors of our stoves to permit the light to shine through; only these are black leaves and those are transparent.” Quite right. What do you call the mineral in your stove door? “Mica, though some people call it isinglass.” Mica is correct. One species of mica is black, and has a particular name; another varies from dark-brown or smoky to transparent, and has a different name. There are also some other species of mica. So you know mica.

Examine this rock very closely—do you find any quartz? “Yes,” you say, “there are two kinds of light-colored minerals here.” Carefully test them both for hardness. Can you scratch them? “Well, no. One of them is hard enough for quartz—it is quartz; but the other I am uncertain about.” Then you must try again. Bear on hard; can’t you make a little scratch with your knife-point, or the end of a file? “I believe I do make a little impression on it.” Well, then, it is not quartz. Now take another look at it. Compare it with the quartz grain by its side. Is its surface broken and irregular? “No,” you say, “it is flat.” Hold it then so as to reflect the light from the window. Is the reflection as bright and glassy as the reflection from the quartz? “I think there is a little difference.” You see, too, it is an unbroken reflection, while that from the quartz is not uniform, in consequence of the uneven surface. There is also another point; this mineral appears to be a fragment of a crystal; you can detect one or more edges or angles. It is not so with the quartz. Thus, in several particulars, this mineral differs from quartz. Its name is Feldspar. But feldspar is not always white nor cream-colored. Very often it is pink-tinted; often almost red. But you may know it to be feldspar by the same signs, independently of color.

So we find in this bowlder three different minerals, and their names are Quartz, Mica, and Feldspar. These three minerals mixed together form the rock Granite. There are several varieties of granite, according to the species of mica; according to the colors of the quartz and feldspar; according to coarseness of the constituents; according to the relative proportions of the three ingredients. But they are all granites. If, however, the minerals are not uniformly mixed; if they are ranged in courses, the rock is stratified, and it is not a proper granite, though quarry men and builders often call it granite. Properly, it is Gneiss (Nice). If the mica is almost or completely wanting in a granite-like rock, the rock is Granulite. When a gneiss-like rock contains very little feldspar, it is Mica Schist (Shist).

Now, let us examine another bowlder, with a similar appearance, but in which the dark mineral is not mica. Be sure, first of all, that we have quartz and feldspar in it. Then, if the dark mineral is not scaly, it is probably Hornblende. It may be nearly black, or greenish-black, or dark green. It may be in grains, or in flat-sided fragments showing an indistinct fibrous structure. It can be scratched, giving a pale bluish-green streak. Now, a rock with these constituent minerals—Quartz, Feldspar, and Hornblende, is Syenite—so called because the rock quarried by the ancient Egyptians at Sye´ne was of this kind. Many persons call this granite also. The “Quincy granite,” near Boston, is a syenite. Often syenite contains also some mica. This is the case with the “obelisk,” in Central Park, New York, and the Mormon Temple, in Salt Lake City. If the constituent minerals tend to arrangement in courses, the rock is stratified, and we call it Syenitic Gneiss. If the quartz is wanting, or nearly so, the rock is Hyposyenite when the feldspar is of the common kind, and Diorite when otherwise. When Syenitic Gneiss contains very little feldspar, we call the rock Hornblende Schist.

The names above explained embrace a majority of the rocks, and I do not by any means expect you to learn at present all of the others. But we may mention a few. Some rocks appear to be composed wholly of one mineral, and yet they are not quartzites. They are mostly dark-colored—slate-colored or blackish or greenish. If these are banded in different colors, or are capable of splitting into sheets, like shingles, they are Argillites—including most of the hard slates, like roofing slate. If a rock is very fine, blackish, and harder than slate, it may be an Aphanite. If it consists of a very fine, hard, uniform reddish or greenish base, having crystals of feldspar scattered through it, the rock is a Porphyry. But, if you feel inclined to go further into rock-details, it will be better to study some work which takes up the subject more thoroughly. (See the Author’s Geological Excursions, and still fuller Geological Studies).

One word more. These bowlder rocks are all hard, crystalline, and generally (not always) foreign to the region where they lie. We sometimes find fragments of rocks which are not hard and crystalline, and far-fetched. They come from ledges which appear at the surface not far away. The most common uncrystalline ledges are of sandstone, limestone, and shale. Sandstone is composed chiefly of grains of quartz—like those in a granular quartzite, but not so brilliant or so firmly compacted together. A grindstone is a fine sandstone. Limestone can be easily scratched with a knife; is generally not composed of grains; and, if you apply strong acid, an effervescence takes place. Very strong vinegar will often suffice, especially if the rock is first pulverized. Shale is quite soft, easily cut with a knife, dark-bluish or black, splitting into thin sheets. It is exceedingly common in coal regions. It crumbles into small fragments, and finally becomes mere mud.

I think this little knowledge about rocks is better than total ignorance. We may now go amongst our bowlders, and speak many of their names.