70 GEOLOGICAL RECONNAISSANCE OF THE DOMINICAN REPUBLIC. 



Zone F. Hard greenish clays with concretions. Corals, Cassis, 

 Zenophora. Approximately 20 



Zone G. Two miles above Los Quemados. Hard grayish-green clays 

 with sandy layers and limonitic nodules. Strata in general unfos- 

 siliferous, but lenses very rich in fossils occur. Amauropsis. Few 

 species in common with the preceding zones. Exposed thickness 

 10 feet; estimated total thickness at least 50 



The Gurabo formation is exposed along Rio Gurabo from near the north 

 end of the stretch of the river shown on Plate XI to a point several kilo- 

 meters above the south end of the stretch shown on the map. For much 

 of this distance the river winds through a narrow gorge cut in the silt- 

 stone of the Gurabo formation. The gorge is narrowest and virtually con- 

 tinuous along the upper reaches of the river. In many places great blocks 

 of coralliferous Mao Adentro limestone have tumbled down from the top 

 of the cliffs and cover the Gurabo formation. 



On Rio Gurabo the top of the Gurabo formation is exposed at the bluff 

 marked "U" (station 8556) on Plate XI, about 3 kilometers south of Gurabo 

 Adentro. The following section at bluff "U" was measured by Mr. Condit : 



Section at bluff "U," Rio Gurabo, about 3 kilometers below Gurabo Adentro. 

 Top of Gurabo formation. Meters. 



4. Calcaieous silt 0.9 



3. Calcareous silt containing many branching corals 0.36 



2. Fossiliferous calcareous silt 1.3 



1. Conglomerate consisting chiefly of pebbles of igneous rock less 



than 75 mm. in diameter 0.46 



The strike of the beds at bluff "U" is N. 60° W., and the dip is 15° NE. 



The strata exposed along Rio Gurabo consist chiefly of beds of fine- 

 grained siltstone. They include a few beds of rather coarse sandstone and 

 some conglomeratic layers, but conglomerate is rare. The prevailing color 

 is light brownish-green, but weathered surfaces are light brown or yellow. 

 The silt in most of the beds is so fine as scarcely to feel gritty when rubbed 

 between the fingers. All the beds are calcareous and some contain cal- 

 careous concretions. Several beds of limestone are intercalated between 

 the silts. Many of the beds of both siltstone and limestone are rich in 

 fossils, but in some beds fossils are scarce or lacking. The pebbles of the 

 conglomerates are of metamorphic rocks, principally igneous, and in part 

 fine-grained and porphyritic. The siltstone contains many glistening 

 particles of a micaceous mineral resembling chlorite. 



Mollusks are less numerous in the Gurabo than in the underlying Cercado 

 formation. Sconsia laevigata is common and appears to be restricted to the 

 Gurabo formation. Corals, both in number of species and in abundance of 

 individuals, are more numerous than in the Cercado formation but less 

 numerous than in the Mao Adentro limestone. Some of the reef-forming 

 corals in the upper ledges of the Gurabo formation are very large ; specimens 



