582 STRUCTURE AND LIFE HISTORIES 



ally become buried in the mud and sediment wherever 

 deposition takes place, and when the deposit becomes 

 converted into rock the organic remains may become con- 

 verted into fossils by either of the processes described 

 above. Swampy regions are especially favorable to the 

 preservation of plant and animal remains as fossils, as is 

 illustrated in Figs. 411 and 412. 



506. Metamorphism. — After sedimentary rocks are 

 once formed they are subject to various changes. The 

 amorphous carbonate of lime, of limestone rocks, may be 

 transformed into crystals of calcite until marble results; 

 thin flakes of mica may form in clay rock in thin sheets, 

 transforming the rock into slate; vegetable deposits in 

 the form of peat may become transformed into anthracite 

 coal and graphite; molten lava poured out on the surface 

 or into crevices of sedimentary rocks may fuse the adja- 

 cent material, causing contact metamorphism; while the 

 heat engendered over larger areas by mountain folding, 

 or by the weight of superincumbent strata^ may cause 

 regional metamorphism. Obviously such changes, espe- 

 cially those caused by heat, result in the complete de- 

 struction of all plant or animal remains or impressions, 

 and thus fossil records over large areas, and representing 

 vast periods of geologic time, have been obliterated. 



507. Stratification of Rocks. — Changes in the relative 

 level of sea and land have occurred many times in the 

 geological past, so that submerged areas of sedimentation 

 in one period have become areas of dry land, undergoing 

 erosion in another; and vice versa, areas of erosion have 

 become areas of sedimentation. As a result of this, 



' Some rocks are buried under more than 40,000 feet of strata, and the 

 temperature increases approximately i°F. for every 50 to 60 feet of depth. 



