HISTORICAL GEOLOGY 



381 



or is under the waters of the seas and so beyond 

 the reach of the geologist. 



Geological Chronology 



Relative ages of strata are determined in two ways. 



(1) Order of Superposition. — If a series of strata 

 or beds is in the order in which they were laid down 

 (Fig. 364), it is evident that the oldest will be at 

 the bottom and the youngest at the top. It is for 

 this reason that the strata of a geological section are 

 always placed with the oldest at the. bottom of the 

 column. This order is conclusive proof of the rela- 

 tive age of rocks unless they have lost their original 

 position by faulting or folding. 



(2) Chronology Determined by Fossils. — After the 

 true order of a series of beds has been determined 

 by their superposition, their contained fossils will 

 usually make it possible to correlate them with 

 strata which may be hundreds or even thousands 

 of miles distant. This is rendered possible by the 

 fact that the inhabitants of the earth have under- 

 gone a progressive change which has, as a whole, 

 been gradual, but which has taken place more rapidly 

 at certain times than at others. Certain classes 

 became dominant for a time, and then declined but 

 seldom entirely disappeared. As a result of this 

 change the assemblage of animals and plants of 

 each division of geological history differs from that 

 of every other. The fact that life has suffered such 

 a progressive modification is of the greatest im- 

 portance, since, as already indicated, it furnishes a 

 means by which the relative age of the rocks in 

 different parts of the world can be determined. 

 Since certain species have a short geological life 

 (their vertical range is short), when such are present 

 the relative age of the rock is readily fixed. 



Although fossils are the surest test of the relative 

 age of widely separated strata it should not be con- 

 cluded that they prove exact contemporaneity, since 

 in favored regions an old fauna may live thousands 



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