OCR Text |
Show THE ORIGIN OF ROCKS. The geologist divided rocks into threo general classes Igneous rocks, originating from the molten magma forced upward from the bowels of the earth, such as granite and the various lavas; sedimentary rocks, consisting of disintegrated particles of the Igneous Igne-ous rocks which have been reformed Into rocks themselves, such as sandstone, sand-stone, and mctamorphlc rocks, consisting con-sisting of igeous or sedimentary rocks whose composition lias been greatly cahnged by Intense heat, chemical action, ac-tion, or other cnuses, as for instance, a limestone that has been changed to marble. Sedimentary rocks, as stated in tho geologic atlas of tho United States geological ge-ological suncy. are usually made up of layers or beds which can be easily separated. These layers are called strata and rocks, deposited in such layers arc termed stratified The surface sur-face of the earth is not immovable, over wide regions it very slowly rises or 6inks with reference to the sea, and , shore lines are thereby changed As a result of upward movement marine se'dimeiitar rocks may become part of the land, and moBt of our land areasaxe. In. fact occupied by rocks orlgfnalh deposited as sediments in tho sea. Sandstone is perhaps the simplest examplcj" of a. sedimentary rock. Obviously Ob-viously sandstone was once sand, and again each grain of sand was once n part of some older rock. This sand was flr3t eroded and then deposited and either subjected to great pressure by overlying masses perhaps thousands thou-sands of feet thick or cemented together to-gether by chemical solutions. Where the stone was formed by simple pressure pres-sure it contains a large proportion of open spaces and thus becomes an Ideal water bearer Some sandstones will carry as much as six quartB of waior to the cubic foot of stone. In other varieties the interstices may havo'-becn filled with cementing ma- ' 'terlal eo that the rock may be for , all practical-purposes almost as dense as granlto |