PHEOORUS DL (OR (TEE aA TH: 33 . SECTLON lh NJ CHAPTER L The Crust of the Earth. 64. Composition of the Crust.—The elementary substances are not equally distributed throughout the earth’s crust. Many of these substances occur only in extremely small quantities, while others are found nearly everywhere. Although the deepest cutting through the earth’s crust does not extend vertically more than about two miles be- low the level of the sea, yet the upturning of the strata, or the outcropping of the different formations, enables us to study a depth of about sixteen miles of the earth’s crust. A careful study of the composition of this part of the crust shows that oxygen constitutes nearly one-half of it, by weight. Silicon, an element which, when combined with oxygen, forms silica or quartz, constitutes, either as sand, or combined with various bases as silicates, one- fourth; so that these two elements form at least three- fourths, by weight, of the entire crust. The following are also prominent ingredients of rocks—aluminium, which, when combined with oxygen, forms alumina, the basis of clay; magnesium, calcium, potassium, sodium, iron, and car- bon. These nine substances, according. to Dana, form Zoyoths, by weight, of the entire crust. Sulphur, hydrogen, chlorine, and nitrogen also occur fre- quently. The remaining elements are of comparatively rare occurrence. 65. The Origin of Rocks.——When the earth was yet a melted globe, the water which now covers the larger portion of its surface hung over it, uncondensed, either as huge clouds or as masses of vapor. After a comparatively thin crust had formed, the vapor was condensed as rain, and cov- ered the earth with a deep layer of boiling water. Occasionally the cooling crust was broken by the increasing tension, and portions of the molten in- terior were forced out: and spread over the sur- face. The muddy waters then cleared by depos- iting layers of sediment over the ocean’s bed. When, by long-continued cooling, the crust be- came thicker, the breaking out of the interior oc- curred less frequently, and contraction, wrinkling the surface in huge folds, caused portions to emerge from the ocean and form dry land. Dur- ing all this time the waters were arranging the looser materials in layers or strata wnieh were ment by water. THE OUTSIDE OF THE EARTH. Ri ~ i ——020300——_ originally more or less horizontal; but wher} ever the contraction forced the melted interior through the crust or upturned it in huge folds, the horizontal position of the deposits was de- stroyed; and even when not so disturbed, the heat of the interior, escaping through fissures, often produced such alterations as to confuse or completely to obliterate all traces of their regu- lar bedding. The almost inconceivable extent of geological time may be inferred from the calculations of Helmholtz, based on the rapidity of the cooling of lava. These calculations show that in passing from a temperature of 2000° C. to 200° C. a time equal to three hundred and fifty million years must have elapsed. Before this a still greater time must have elapsed, and after it came the exceedingly great ex- tent of geological time proper. 66. According to their Origin, rocks may be divided into three distinct classes: (1.) Igneous Rocks, or those ejected in a melted condition from the interior, and afterward cooled. (2.) Aqueous Rocks, or those deposited as sedi- When mineral matter settles in water, the coarser, heavier particles reach the bot- tom first, 80 that a sorting action occurs, which makes the different layers or strata vary in the size and density of their particles, and, to a great extent, in their composition. Aqueous rocks are sometimes called sediment- ary rocks. (3.) Metamorphic Rocks, or those originally deposited in layers, but afterward so changed by the action of heat as to lose all traces of stratifi- cation. This change, which is called metamorphism, is caused by heat acting under pressure in the presence of moisture. Under these conditions a far less intense heat is required to re- move all traces of stratification. Metamorphism appears to consist mainly in a rearrangement of the chemical con- stituents of the rocks, 67. According to their Condition, rocks may be divided into two classes: (1.) Stratified Rocks, or those arranged i regular layers. Aqueous rocks are always ian fied, and sometimes, though rarely, metamorphic ori are stratified.