120 EVOLUTION AND GENETICS 



the bottom as they died, and the calcareous parts of their bodies 

 formed other layers. Through long periods of time these layers 

 of sediments were transformed into rocks, — shales, sandstones and 

 limestones, respectively, — which are called sedimentary rocks in 

 contrast to the basaltic and granitic igneous rocks formed by the 

 cooling of molten masses. 



Succeeding readjustment of the unstable crust resulted in the 

 elevation of these stratified sedimentary rocks above the waters 

 in which they were laid down. They were repeatedly folded and 

 broken by great cataclysms which left jagged remnants projecting 

 thousands of feet above the level of the surrounding land masses, 

 and thus formed our mountain ranges. When once elevated, the 

 sedimentary rocks themselves were exposed to the forces which 

 weather and erode, and were in turn cut away and carried down to 

 the sea to play their part in the formation of new sedimentary 

 strata lying upon those previously deposited. 



Geological Time. The lengths of time involved in this succes- 

 sion of constructive and destructive processes is inconceivable, 

 but some slight comprehension is possible when we consider the 

 enormous thickness of the aggregate deposits and the slowness with 

 which materials are now being removed from the land masses. 

 In all, geologists estimate sedimentary deposits to have reached a 

 mean thickness of fifty-three miles. As Schuchert graphically 

 expresses it, "this means the more or less rapid wearing away 

 almost to sea-level, one after another, of more than twenty ranges 

 of mountains like the present European Alps or the American 

 Rockies. During the incredibly long intermediate time, when the 

 lands were planed to a low relief, there was very little erosion." 

 And yet the continent on which we live is said to be undergoing 

 denudation to the extent of only one foot in 8,600 years. Since the 

 earliest record of history, North America has lost, according to 

 these figures, less than a foot of altitude on the average, yet the 

 strata found in the Appalachian mountains indicate that they 

 once reached twenty thousand feet above sea level. More than 

 fifteen thousand feet of rocks worn away and washed into the sea 

 at a rate which is imperceptible within human experience! 



Even in view of the fact that erosion proceeds at a much more 

 rapid rate where rainfall is abundant and slopes are steep, these 

 enormous movements of material prior to the coming of modern 

 man are impressive. The Appalachian Mountains are the same 



