28 A NATURALIST IN THE GREAT LAKES REGION 



again. The deeper layers of those vast accumulations of sedi- 

 ment in ocean deeps, in inland seas, and great bays were under 

 the terrific pressure of the overlying layers and fathoms of 

 ocean waters. They became intensely heated, for they were far 

 enough below the surface to feel the heat of the earth's interior, 

 retained by the blanket of overlying layers. So they were trans- 

 formed into rock. 



These beds raised the temperature of the older underlying 

 rocks almost to the point of fusion, establishing a line of weak- 

 ness here so that as the crust movements, due to unequal shrink- 

 age, occurred these areas of weakness were readily upheaved, the 

 sedimentary rock strata were arched, sometimes crumpled, and 

 thrown up above the sea. 



Similarly all over the earth the sedimentary rocks, formed in 

 successive geological ages, have been upheaved and occasionally 

 exposed to our inspection. These rocks often contain fossils, 

 the remains of animals, and plants that lived in past geological 

 times. Such animals and plants living in the water or washed 

 from the land into the streams possibly by spring freshets were 

 carried seaward and deposited with other debris, with sand and 

 silt in the great mud banks in lakes and oceans. These, by the 

 process outlined above, were changed to stone, and so the 

 bones or shells of animals, the bark, fruits, and even leaves of 

 plants have been preserved, gradually assuming a stony texture 

 (Fig. 34, 36). Or if the softer parts rotted away, the cavity 

 left filled in with rock in time and so made a cast of the organism 

 showing to this day all the delicate details of the original. These 

 fossil remains, when carefully studied, yield an interesting history 

 of living things upon the earth. The .science which deals with 

 these fossil remains is paleontology, a special department of 

 geology. 



Our western plains are, in places, particularly rich in the 

 fossil remains of some of the great vertebrates. Scarcely a 

 summer passes that scientific expeditions are not organized to 

 go out and explore these regions and find the fossils. The 



