THE GEOLOGICAL BACKGROUND OF EVOLUTION 391 



particularly important in studying the rocks of folded mountains, where 

 the strata may be turned up into steeply tilted positions or even some- 

 times folded completely back on themselves, so that locally they may 

 lie in reverse order — younger strata beneath older ones. 1 



The history of the continents. The results of studies that cannot 

 here be discussed show that the continents and ocean basins have existed 

 since the veiy beginning of that part of earth history recorded in the rocks. 

 The ocean basins are a little more than brimful of water, so that the 

 edges of the continents are generally covered by shallow seas. Such seas, 

 resting upon the continents, are called epicontinental seas. 



Geological cycles. On every continent a certain cycle of events has 

 repeated itself many times with slight variations. At the beginning of a 

 typical geologic cycle, the continent stands high; the epicontinental seas 

 are restricted to the edges; and the high lands are being rapidly eroded 

 by swift -flowing streams that carry their sediments to or over the edges 

 of the continental shelves. As the lands are degraded and the sediments 

 are deposited in the seas, the water level rises and slowly creeps in over 

 the low parts of the land. Hudson Bay appears to be an example of such 

 an advancing sea. By the middle of the cycle the lands are worn low, and 

 the interior of the continent is occupied by widespread, shallow interior 

 seas, in which are deposited the sediments from the remaining land areas. 

 With warpings of the continent, the seas slowly fluctuate in form and 

 extent, but they occupy the land for periods of one to several million 

 years. Finally, toward the end of the cycle, readjustment sets in. The 

 continents rise, often with the formation of a mountain range in some part 

 where the sediments were thickest, the seas are spilled off, and the cycle 

 is ready to repeat itself. 



This cyclic series of events forms the basis for the subdivision of earth 

 history into periods of time, corresponding to the systems of strata 

 formed during the periods of sea invasion. Each spread of the sea left 

 strata of rock on top of those formed during the previous cycles. There 

 are breaks in the series, due to the times when the continent stood high. 

 During these times there was no deposition of sediments on the conti- 

 nents, and streams were busy destroying the earlier depositions. These 

 breaks (unconformities) in the rock record also appear to be breaks in the 

 story of life, for the marine forms at least. Life was continually changing, 

 and at each return the seas brought with them a fauna that was different 

 from that of the previous cycle. 



Geological revolutions. On the whole these cycles have affected all 

 the continents simultaneously, so that the divisions of the rock record in 

 one region hold good for other parts of the world. But there are generally 



1 Unfortunately, where such disturbance of the strata has been greatest, the fossils 

 have been destroyed, and other methods of correlation must be used. 



