78 GEOLOGICAL RECONNAISSANCE OF THE DOMINICAN REPUBLIC. 



Except within the basal complex, where folding as well as faulting has 

 been intense, faulting is the dominant element of the structure and folding 

 is distinctly subordinate. Faults outline many of both the major and minor 

 topographic features and determine the distribution of the geologic forma- 

 tions. The three longitudinal valleys that cross the island — the Cibao 

 Valley, the San Juan-Azua area, and the Enriquillo Basin — are areas of 

 Miocene and younger sediments that are partly outlined by faults. The 

 mountain ridges that bound these valleys are composed of hard, older for- 

 mations. Block faulting, both in the Cordillera Central and in the flank- 

 ing areas, has caused frequent repetition of strata and greatly complicates 

 the problem of interpreting the stratigraphy of the island. 



The structure of the southwestern part of the Cibao Valley is monoclinal. 

 The Miocene formations of the Yaque group dip gently northward and abut 

 against the steep front of the Cordillera Septentrional, which, in the area 

 between Santiago and Esperanza, is a fault scarp. At places, as in Arroyo 

 las Lavas at the crossing of the Monte Cristi road, the edge of the Miocene 

 formations has been dragged upward and the strata slightly overturned. 

 At Damajagua, northwest of Navarrete, this great fault brings up Creta- 

 ceous "hornstone" and Eocene limestone high above the Miocene. The 

 extension of this fault toward the northwest and southeast has not been 

 traced. No faults of great magnitude were observed near Monte Cristi. 

 Faulting has also taken place along the south side of the Cibao Valley, but 

 the throw of the faults in that area is apparently not so great as that above 

 described. The Oligocene Tabera formation, which dips steeply away 

 from the Cordillera Central, is cut by many faults, but the throw of all 

 that were examined is small. Block faulting of greater magnitude has 

 taken place in the neighborhood of San Jose de las Matas and also near 

 Dajabon. 



The east end of the Cibao Valley is a gentle syncline. The Cevicos lime- 

 stone in the neighborhood of Cevicos dips very gently northward, and the 

 yellowish limestone that forms the foothills of the Cordillera Septentrional 

 near Villa Rivas and Arenoso, which probably should be correlated with 

 the Cevicos limestone, is inclined toward the south. No faulting was ob- 

 served along the northern margin of the Cibao Valley east of San Francisco 

 de Macoris. The large fault that forms the scarp near Navarrete either 

 does not extend so far east or passes north of any route traversed by a 

 member of the expedition. 



Faults of undetermined magnitude involve schists and associated 

 rocks of the basal complex and also hard limestones, probably of Eocene 

 age, in the Lomas de Sierra Prieta near the margin of the Cibao Valley 

 west of Rio Yuna and at Hatillo near Rio Yuna, southwest of Cotui. 



The mountain front north of Azua marks a fault which brings limestones 

 and shales, probably Eocene or older, against beds of Miocene age. This 



