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. 
606. 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 stale; 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 melamorphism; while the 
heat engendered over larger areas by mountain folding, 
or by the weight of superincumbent strata 1 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, 
1 Some rocks are buried under more than 40,000 feet of strata, and the 
temperature increases approximately iF. for every 50 to 60 feet of depth. 
