PALEONTOLOGY 293 



Many rocks are, like lava, of igneous origin; others, like 

 granite, although not originally in melted condition, have been 

 so heated subsequent to their formation, that any traces of 

 animal or plant remains in them have been obliterated. Fossils 

 are found almost exclusively in rocks wliich have been formed 

 by the slow deposition in water of sand, clay, mud, or lime. 

 The sediment which is carried into a lake or ocean by the 

 streams opening into it sinks slowly to the bottom of the lake 

 or ocean and forms there a layer which gradually hardens under 

 pressure to become rock. This is called sedimentary rock, or 

 stratified rock, because it is composed of sediment, and sedi- 

 ment always arranges itself in layers or strata. In sedimentary 



Fig. 174. — Restoration of the skeleton of Dimorphodon macronyx. (After Seeley.) 



or stratified rocks fossils are found. The commonest rocks of 

 this sort are limestone, sandstone, and shales. Limestone is 

 formed chiefly of carbonate of lime; sandstone is cemented 

 sand; and shales, or slaty rocks, are formed chiefly of clay. 



The formation of sedimentary rocks has been going on since 

 land first rose from the level of the sea; for water has always 

 been wearing away rock and carrying it as sediment into rivers, 

 and rivers have always been canying the worn-off lime and 

 sand and clay downward to lakes and oceans, at the bottoms 

 of which the particles have been piled up in layers and have 

 formed new rock strata. But geologists have shown tliat in 

 the course of the earth's history there have been great clianges 

 in the position and extent of land and sea. Sea bottoms have 

 been folded or upheaved to form dry land, wliile regions, once 

 land, have sunk and been covered by lakes and seas. Again, 

 through great foldings in the cooling crust of the earth, which 

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