GEOLOGY AND GEOLOGIC HISTORY. 79 



fault appears to run nearly due east and west and intersects the north- 

 south fault which extends from San Jose de Ocoa southward along the 

 edge of Loma el Numero. Rio Ocoa from San Jose de Ocoa to Los Ran- 

 chitos follows this north-south fault line. 



The diastrophic movements that have affected the Dominican Republic 

 occurred during many geologic periods. The stresses that folded and 

 sheared the rocks of the basal complex and converted so many of them into 

 schists were probably active during Eocene time or earlier. Great diastro- 

 phic movement probably antedated later Eocene time, when limestones 

 were formed over much of the present land surface of the island. The 

 Tabera formation, of middle Oligocene age, was steeply tilted, faulted, and 

 uplifted during late Oligocene time, while the upper Oligocene Cevicos lime- 

 stone was being laid down in a sinking area farther east, which may have 

 remained above water while the shales and conglomerates of the Tabera 

 formation were deposited. The latest known Miocene deposit, the Cerros 

 de Sal formation, has in places been faulted and tilted vertically since its 

 deposition. The deposits of salt and gypsum in the Cerros de Sal forma- 

 tion show that changes in the level of land and sea were taking place while 

 the beds were being deposited. Gentle flexures in the Las Matas forma- 

 tion, which is supposed to be of Pliocene age, and many raised beaches and 

 uplifted coral reefs not older than Pleistocene record oscillations of compar- 

 atively recent date. Nor is there any sign of cessation of crustal movements 

 in this region. On the contrary, evidence on every hand indicates that 

 crustal movements are still in progress. The violent earthquakes which 

 have repeatedly laid waste the ancient city of Santiago link the diastro- 

 phism of the past with that of the present and indicate that crustal move- 

 ments are probably going on today with as great intensity as at any time 

 in the past. 



OUTLINE OF GEOLOGIC HISTORY. 



The geologic history of the Dominican Republic is not yet fully under- 

 stood and any outline of it must therefore be defective. The oldest rocks 

 whose age has been definitely established are Upper Cretaceous deposits, 

 but these deposits contain pieces of igneous rock, which indicate either 

 Cretaceous or earlier igneous activity. Contemporaneous igneous activity 

 is shown by the interbedding of extrusive igneous rocks with Cretaceous 

 sediments. As the Cretaceous sediments are of a kind characteristic of 

 shallow water, there was necessarily a shoal-water bank over at least 

 a part of the area now occupied by the Dominican Republic. Some of 

 the rocks may be much older than Upper Cretaceous. In Cuba Doctor 

 Barnum Brown has recently traced an Upper Jurassic shore line, showing 

 that there was land in Cuba prior to Upper Jurassic time. Farther west, 

 in Guatemala and Nicaragua, certain prominent tectonic features that are 



