80 GEOLOGICAL RECONNAISSANCE OF THE DOMINICAN REPUBLIC. 



in line with the main tectonic features of the Antilles existed at the close 

 of Paleozoic time. In fact, a gradually increasing body of evidence points 

 to the existence of the major tectonic lines of the Antilles in late Paleozoic 

 time. 



It is inferred that during early Eocene time the site of the Dominican 

 Republic was mostly a land area, because the widespread deposits of Eocene 

 age all seem to belong to the latest subdivision of Eocene time. Until 

 the geologic map of the Republic has been completed it will not be practica- 

 ble to ascertain whether any part of it remained above sea during all of 

 Eocene time, but the Eocene sediments contain large Foraminifera, such as 

 Lepidocyclina, Orthophragmina, and Nummulites, that inhabited shallow 

 tropical waters. The limestones in the areas occupied by Sierra de Neiba, 

 Sierra de Bahoruco, and other mountains in the southwestern part of the 

 Republic attained considerable thickness, and Eocene limestone with con- 

 clomerate at its base overlies the Cretaceous deposits in the Cordillera 

 Septentrional. The Eocene limestones, however, appear to be thicker in 

 Haiti than in the Dominican Republic. The intrusion of the great masses 

 of dioritic rocks probably occurred before the deposition of the Eocene 

 sediments. 



It seems that after the deposition of the Eocene sediments there was a 

 period of diastrophism, because the Tabera formation, of middle Oligocene 

 age, is composed largely of coarse conglomerate, which indicates erosion at 

 its source, and the Oligocene deposits in the vicinity of Los Bancos appear 

 to be separated from Eocene deposits by a marked erosion unconformity. 

 (See p. 95, Chapter VI.) Before middle Oligocene time there must have 

 been a great deal of igneous activity to supply the material for the conglom- 

 erates and there may have been volcanic extrusions. Igneous activity dur- 

 ing Oligocene time is shown by the interbedding of tuffs with fossiliferous 

 deposits. Although middle Oligocene deposits occupy considerable areas 

 in the Republic the whole of its surface was not under sea at this time. The 

 earth movements during this period were evidently complex, and at the 

 close of Oligocene time there appears to have been further diastrophic 

 movement. 



At the beginning of Miocene sedimentation the central axis of the Island 

 was a mountainous ridge bordered by a shallow sea in which the basal con- 

 glomerate and sandy sediments of the Cibao Valley and of Azua Province 

 were deposited to a thickness ranging from a few hundred to at least 1,500 

 meters. Active erosion of the adjacent land was almost continuous, and 

 sandy sediments were spread over the sea bottom with only a temporary 

 pause now and then permitting the deposition of thin beds of limestone. 

 Nearly all the calcareous beds are made up of reef corals intermixed with 

 some sandy material. No evidence of igneous activity in Miocene time 

 is now known. 



