232 GEOLOGICAL RECONNAISSANCE OF THE DOMINICAN REPUBLIC. 



GEOGRAPHY. 



Sierra Prieta is an isolated outlier of the main mountain range about 

 30 kilometers north of the city of Santo Domingo and about 12 kilometers 

 northeast of Los Alcarrizos, the nearest point on the automobile road 

 (Carretera Duarte) now being constructed from the capital city northwest- 

 ward across the island. A direct trail, passable by light automobiles in 

 dry weather, leads from Santo Domingo City northward through Villa 

 Mella. It is an easy, natural route, which with little work could be made 

 into a good wagon road, following as it does the nearly level upland surface 

 of the "coast limestone," with only the valleys of Rio Isabela and Rio 

 Yuca offering serious obstacles. Mr. Leslie, agent for the company, 

 reports that a railroad route to the mine, extending in part up Rio Ozama, 

 has already been surveyed. The altitude of the plain near the foothills of 

 the mountain range is 70 to 80 meters. Sierra Prieta, the hill containing 

 the nickel deposits, rises abruptly from this plain to an altitude of 241 meters 

 and about 170 meters above the plain to the south. The rugged upland 

 country of which this hill is an outlier extends northward to the Vega 

 Real. 



GEOLOGY. 



The coast at Santo Domingo consists of a coral reef that has been elevated 

 about 15 meters above sea level. Landward of this reef is a nearly level 

 surface, which rises toward the north as a plain that extends to the moun- 

 tains. This plain is made up of strata that slope gently seaward. The 

 beds farthest from the coast are the oldest and are overlapped by succes- 

 sively younger rocks seaward. Coralliferous limestone, probably not all 

 of the same age as that at the sea front, extends a few kilometers north 

 of Santo Domingo, but beyond this limit the sections in the arroyos show 

 chiefly gravel, sand, and fossiliferous nodules of limestone. Nearer the 

 mountains the plain has a thin covering of limonitic conglomerate, which 

 toward the foothills thickens and becomes increasingly ferruginous and 

 might be of possible value as iron ore. The mountains in this vicinity con- 

 sist of serpentine, which forms part of a large belt that extends northwest- 

 ward across to the vicinity of La Vega. The stream pebbles are of slate 

 and limestone and of many varieties of intrusive rock. The hill containing 

 the nickel deposit is entirely serpentine, as is also the neighboring hill to the 

 north, and no other rocks were found in place except a small basic dike 

 that cuts the serpentine. 



Gold has been washed by the natives from the stream gravels of the 

 region and it is said that small quantities of platinum have been found in 

 the placers, but probably not in quantities sufficient to encourage hopes of 

 profitable working. A sample of black sand washed from a small brook 

 near the camp at the east base of the hill was found on examination to con- 

 tain chromite in addition to considerable magnetite. 



