176 GEOLOGICAL RECONNAISSANCE OF THE DOMINICAN REPUBLIC. 



STRUCTURE. 



The structure of the rocks under the lower part of the Yaque Valley 

 has an important bearing on possible supplies of underground water. 

 The structure of the beds in this region is not a result of deformation during 

 a single period but was developed progressively ; each one of the sedimentary 

 formations described above was deformed before the next younger forma- 

 tion was deposited. There has been both folding and faulting, but the 

 deformation was not so intense as to produce great dynamic metamor- 

 phism. The older formations are somewhat metamorphosed, but the 

 younger ones have been very slightly affected. 



On the south side of the valley are the foothills of the Cordillera Central, 

 which are evidently fault blocks. The quartz diorite appears abruptly in 

 one of the foothills in Juan Calvo Hills near Dajabon, and there is a distinct 

 shear zone. The topography of other outlying hills, Cerros de Jacuba, 

 suggests faulting, and the correctness of this view is supported by the pres- 

 ence in them of limestone apparently of Oligocene age. 



On the north side of the valley, along the southern foot of the Monte 

 Cristi Range, no indications of faults of great magnitude were observed, 

 but in some places there has been folding sufficient to produce dips as high 

 as 70°. The rocks that underlie the valley were seen also in the Monte 

 Cristi Range, where they are more disturbed than in the valley, and at 

 Barranca Blanca Miocene strata have been faulted. The strike of the struc- 

 tural lines is prevailingly east and west. A study of El Morro de Monte 

 Cristi (see section on p. 173) shows the presence there of strata equivalent 

 to the Gurabo formation between altitudes of 153 and 155 meters above 

 sea level, and the hill is 57 meters higher. Near La Barca, on the south side 

 of Rio Yaque opposite the town of Monte Cristi (station 8780, p. 154), 

 fossils representing either the upper part of the Gurabo formation or the 

 Mao Adentro limestone were collected about 5 meters above sea level. 

 The bed at this station represents a horizon stratigraphically as high as 

 or higher than the horizon of the bed exposed in El Morro de Monte 

 Cristi at an altitude 150 meters or more higher topographically. The dip 

 on the Morro is 5° N., but the beds at station 8780 are almost undisturbed. 

 These relations imply considerable deformation and perhaps a fault along 

 or near the south side of the Morro, with downthrow to the south and 

 upthrow to the north. The observations here recorded invalidate the 

 opinion of Gabb and Heneken, quoted by Maury, that the limestone cap- 

 ping the Morro is "a continuation of that capping the Samba Hills, the 

 intervening part having been removed by denudation." l 



The floor of the lower part of the Yaque Valley is formed of strata of 

 Miocene age, which are either exposed or lie under a thin cover of younger 

 deposits. These strata are slightly inclined, crossed by gently undulatory 



1 Maury, Carlotta J., Bull. Am. Paleontology, vol. 5, p. 450, 1917. 



