STRATIGRAPHIC PALEONTOLOGY. 91 



GEOLOGIC CORRELATION. 



The expression "geologic correlation" means the determination or the 

 attempt to determine the contemporaneity of geologic events in different 

 regions — proximate or remote — the attempt to determine, for instance, 

 what deposits were forming at the same time at the two ends of the present 

 valley of Rio Yaque del Norte and what deposits were forming at that time 

 on the south side of the Cordillera Central. As applied to the Dominican 

 Republic it involves a comparison of the geologic history of the Dominican 

 Republic with that of Cuba, Jamaica, and other West Indian islands, and 

 of the geologic history of this general region with that of other parts of 

 America or of more remote parts of the earth. 



Geologic correlation may be made: (1) by actually tracing geologic 

 formations from one area to another through adjacent exposures; (2) by 

 comparing the record of the diastrophic events (earth movements) in the 

 two areas; (3) by studying the remains of such organisms as may be en- 

 tombed in the sediments in the two areas if they are sufficiently abundant. 

 Each of these methods has its limitations and its liability to error, and in 

 many regions a reliable conclusion can not be reached by means of any one 

 of them, but satisfactory correlations in some regions may be reached by 

 combining evidence afforded by two or all of them. 



Correlation by tracing formations through lithologic continuity and simi- 

 larity is not invariably trustworthy, because formations that are different 

 in character may have been deposited contemporaneously and because 

 similar formations may have been deposited at different times. Everyone 

 who is familiar with the physical features of the Republic knows that the 

 sediments now forming at the mouths of such streams as Rio Yuna and Rio 

 Yaque del Norte are different from those forming on Monte Cristi Bank or 

 in any other area around the shores of the Dominican Republic that is not 

 reached by detritus washed from the land; yet these different kinds of 

 sediments are forming at precisely the same time. One could not therefore 

 correlate these contemporary river-borne formations by tracing them along 

 the shore from the mouth of the Yuna to the mouth of the Yaque, for they 

 are not continuous. The discontinuity between the formations of Haiti 

 and of Cuba is still more striking. Rock of the same kind may be formed in 

 widely separated geologic ages. In the West Indies there are shallow- 

 water limestones of Jurassic, Upper Cretaceous, Eocene, middle Oligocene, 

 late Oligocene, Miocene, and Pleistocene age, so that one who employed 

 no other criteria than lithology for determining the geologic age of a de- 

 posit — which means also its stratigraphic position — might make very 

 serious errors. Yet, although geologic correlations by lithology may some- 

 times fail, it is nevertheless valuable. 



