92 GEOLOGICAL RECONNAISSANCE OF THE DOMINICAN REPUBLIC. 



Correlations by diastrophic events, a method which is based on similarity 

 in deformation or in succession of deformation or on similarity in degree 

 of metamorphism, is also defective, for the whole crust of the earth is not 

 simultaneously subjected to earth movements of the same kind and inten- 

 sity. Parts of the earth's crust have remained almost undisturbed while 

 other parts have been folded into mountain chains, and igneous activity 

 was violent in some regions while there was none at all in others. Not- 

 withstanding the limitations of the method of correlation by diastrophism, 

 however, it is of great value, because the strata in some large parts of the 

 earth did undergo deformation at nearly the same time, and in a study of 

 those parts the complicated events of the past may be traced by carefully 

 studying the relations of the rocks one to another and thereby learning 

 something of the history of their deformation. 



Correlation by means of fossil organisms depends upon the premise that 

 the organisms which inhabited the earth in past geologic time have changed 

 in character and in grouping from age to age and that the organisms which 

 lived in each geologic age were similar. The defects of this method of cor- 

 relation are numerous: (1) Many deposits, particularly those formed on the 

 land, contain no organic remains; (2) land, water, or climatic barriers may 

 not permit organisms to move freely to any part of the earth — that is, the 

 organisms of one time and place may be restricted in their geographic dis- 

 tribution, and the organisms that lived at a certain time may have been 

 very different in different regions; (3) if organisms had suddenly changed 

 at the beginning of each geologic epoch and if all the organisms that lived 

 during that epoch were characteristic of it there would be no great difficulty 

 in recognizing the age of a deposit that contains abundant organic remains, 

 but abrupt breaks do not generally occur between faunas of successive 

 epochs. For instance, it seems that nearly all the species of the living 

 shallow-water coral fauna of the West Indies already existed in Miocene 

 time — probably in older Miocene time. The Miocene coral fauna of the 

 West Indies is characterized by some genera and groups of species some of 

 which are now extinct while others persist in the Indo-Pacific region. 

 Almost all the superspecific groups of the Recent West Indian shallow-water 

 Mollusca were also living in Miocene time in the West Indian region, but 

 the Miocene Mollusca include groups that are now restricted to the west 

 coast of America or to the Indo-Pacific region. 



Correlation by means of fossils involves still other difficulties and lia- 

 bilities to error. The original subdivision of the Tertiary system into 

 Eocene, Miocene, and Pliocene was made by Lyell according to the per- 

 centage of living species of mollusks found in the rocks, the Eocene con- 

 taining 3.5 per cent, the Miocene 17 per cent, and the Pliocene 30 to 50 

 per cent of species still living. Two other subdivisions of the Tertiary 

 were afterward made, the Oligocene by Beyrich and the Paleocene by 



