HOW ROCKS WERE FORMED 



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199 



solidified from a molten state are classed as igneous 

 rocks. 



Rocks which are made of sediment cemented to- 

 gether are called sedimentary rocks. Much of the pres- 

 ent land area of the United States was at one time 

 covered by a sea or an ocean. Streams deposited their 

 sediment at the bottom of the sea just as they are 

 doing today. In time these sediments were changed 

 to rock. If a rock consists of particles of sand ce- 

 mented together it is called sandstone. Rocks formed 

 from gravel deposits are called conglomerates. Rocks 

 formed from clay are called shale. Limestone is also 

 sedimentary rock, believed to have been formed in 

 many cases from the shells and skeletons of ocean 

 animals. Limestone, sandstone, and shale are the lead- 

 ing types of the great group of sedimentary rocks. 

 Try to find a specimen of each type. 



Examine several specimens of sedimentary rock to 

 see whether they are marked by parallel lines. Do 

 any of them seem to be made up of thin layers? Is 



Keystone View Co. 

 FIG. 321. LIMESTONE ROCKS AT MAMMOTH CAVE, KENTUCKY 



each layer spread evenly over another? Rocks of this 

 type are called stratified. Each layer is known as a 

 stratum (plural, strata). Stratification can be seen 

 very well in soft coal, a sedimentary rock made from 

 vegetable matter. 



Sedimentary or igneous rocks that have been 

 changed by the combined activities of heat, pressure, 



ANIMALS THAT LIVED 25,000,000 YEARS AGO 



ANIMALS THAT LIVED 200,000,000 YEARS AGO 



ANIMALS THAT LIVED 400,000,000 YEARS AGO 



ANIMALS THAT LIVED 550,000,000 YEARS AGO 

 FIG. 322. ANCIENT LIFE AS RECONSTRUCTED FROM FOSSILS 



and chemical action become metamorphic rocks. Mar- 

 ble is a metamorphic rock formed from limestone, a 

 sedimentary rock. Slate is a metamorphic rock formed 

 from shale. Gneiss (nls) is a metamorphic rock formed 

 either from granite, an igneous rock, or from con- 

 glomerate, a sedimentary rock. 



Metamorphic rocks resemble igneous rocks in that 

 they frequently have a crystalline structure. Many 



