74 GEOLOGICAL RECONNAISSANCE OF THE DOMINICAN REPUBLIC. 



Obras Piiblicas for road metal. The rock is yellowish, soft, somewhat argil- 

 laceous coralliferous limestone, in which are lumps of harder limestone 

 composed largely of corals and other fossils. The harder lumps are used as 

 road metal and the soft matrix is discarded. The rock is exposed in numer- 

 ous test pits scattered over the hills. In a quarry where a face over 30 

 meters wide is exposed a good section of the formation was observed. Here 

 the lumps suitable for road metal occur in a bed about 3 meters thick that 

 strikes approximately N. 30° E. and dips 15° N. This bed of harder 

 material is underlain by calcareous clay, which is about 6 meters thick and 

 contains a few small, hard lumps. Below the calcareous clay is an equal 

 thickness of bedded sand, which has been disturbed by a small fault. The 

 sand contains a few small coral lumps in the upper part and a few thin beds, 

 about 5 centimeters thick, of hard, impure limestone in the lower part. 



The Mao Adentro limestone is an important formation in the valley of 

 Rio Yaque del Norte, for it seems to form the summits of most of the promi- 

 nent hogback ridges there. Besides the ridges already mentioned there is 

 Sierra del Viento, which is north of Rio Yaque and east of Rio Amina. This 

 ridge is capped by a hard coralliferous limestone, which is underlain by an 

 argillaceous or arenaceous deposit containing great numbers of branching 

 corals. The beds in which branching corals are so numerous are ten- 

 tatively considered a part of the Gurabo formation. 



As the Gurabo formation appears to grade into the Mao Adentro lime- 

 stone, notwithstanding the presence of beds of gravel near their contact 

 and evidence of crustal disturbance in the area whence the gravels were 

 derived, it was not practicable to decide with certainty as to which of the 

 two formations some exposures should be referred. The Mao Adentro 

 limestone probably possesses a peculiarity that will need to be considered 

 in future field work. It is composed largely of corals, and in places it seems 

 to be true coral-reef rock. Living coral reefs form broken ridges or chains 

 or discontinuous patches of limestone, between and on the sides of which are 

 contemporaneous sediments of different character. It is highly probable 

 that the Mao Adentro limestone never formed a continuous rock sheet 

 throughout its extent, but that it was composed of broken ridges and 

 patches similar to the coral reefs of modern times. The relations of this 

 limestone to older, contemporaneous, and younger deposits offer puzzling 

 problems that can be solved only by detailed field studies. 



The fossils obtained in the Mao Adentro limestone are listed on pages 

 152-153. 



MAO CLAY. 



The Mao clay overlies the Mao Adentro limestone and forms the top- 

 most division that has thus far been discriminated in the Yaque group. It 

 is exposed in a bluff on the west side of Rio Mao 2 or 3 kilometers south of 

 the town of Valverde (old name Mao). The sediment exposed in this bluff 



