388 



THE CHANGING GENERATIONS 



been gradual, or that past events must be interpreted in the light of what 

 is going on today. These conclusions apply to evolution as well as to 

 geology. Uniformitarianism does not mean, however, that the past was 

 no different from the present or that the rate of geological and evolution- 

 ary change has remained constant. There was a time when the earth 

 was molten and no life existed. Even since it has cooled and animals and 

 plants have appeared upon it, the earth has undergone more or less cyclic 

 physical changes because of which the rate of geologic and of evolutionary 

 processes has varied from time to time. 



The rocks of the earth's crust. The outer crust of the earth is a shell 

 of rock about 60 miles thick, crystalline in its outer 20 miles or so and 

 gradually becoming glasslike toward its lower limit. Beneath it are other 

 denser layers surrounding the supposedly metallic core. Distributed 

 about the surface are the continents — great masses of lighter rock floating 

 low in the denser and heavier rock that makes up most of the crust. The 

 places where the lighter rock is thin or absent are the ocean basins. The 

 light crystalline rock of which the continents are chiefly made is granite, 

 formed, like other kinds of crystalline rock, by cooling and crystallization 

 of molten rock liquids similar to lava. Rocks formed in this way are called 

 igneous 1 rocks, because heat was involved in their origin. Igneous rocks 

 can obviously never contain fossils. 



The land surfaces are exposed to the action of air, moisture, and changes 

 of temperature, and to the influences of animals and plants. The com- 

 bined effect of these agencies is to cause the superficial layers of rock to 

 "decay" and disintegrate, a process termed weathering. The loose mate- 

 rials thus formed blanket the surface; they are blown by the wind, washed 

 down slopes by the rain, and carried off by streams. Where the streams 

 empty into lakes or the sea, the finely divided rock material (sediment) 

 settles to the bottom and forms a layer of mud or sand. In the course of 

 time earlier layers are covered by later ones, which in their turn are 

 buried, until the accumulation may become very deep. Compacted by 

 the weight of the overlying materials and cemented by slow chemical 

 changes, the layered sediments harden into solid rock. Rocks thus formed 

 are called stratified rocks in reference to their layered structure, or sedi- 

 mentary rocks because they are formed from sediments. 



The continents are almost entirely covered by overlapping sheets of 

 sedimentary rock. Some of the sheets are very thin, and some very thick 

 in places; some are local, but many extend over areas of hundreds or 

 thousands of square miles. The widespread deposits bear evidence of 

 having been formed in shallow seas that lay upon the surfaces of the 

 continents, and they represent the mud and sand poured into the seas 

 by rivers that flowed off the bordering lands. Once they were the muddy 



1 From the Latin, igneus, "fiery." 



