72 GEOLOGICAL RECONNAISSANCE OF THE DOMINICAN REPUBLIC. 



Section on left bank of Rio Mao 1 kilometer above the ford at Cercado de Mao. 



Pleistocene: Meters. 



4. Thin cover of river gravel, thickness variable. 

 Miocene (Gurabo formation): 



3. Soft yellowish fine silty marl containing Placocyathus, Amu- 

 sium, and Amphistegina. Contains a few harder nodular 

 ledges, especially in the lower part. Forms a vertical or 



overhanging bluff 12.2 



2. Thin-bedded hard yellowish limestone. Some ledges are 



platy. Collection cf corals. Station 8520 9.1 



1. Massive soft calcareous sand with a few locally indurated 

 nodular ledges. Contains many mollusks and corals, 

 especially in the lower part. "Sconsia laevigata zone." 

 Station 8519 27.4 



The bluff on the right bank of Rio Mao opposite Cercado de Mao, about 1 

 kilometer below Paso Bajito, the ford of the road to Potrero, shows about 

 12 meters of blue clay, light gray when dry, stained yellow with iron in 

 places. This bed (station 8527) , which contains many fossil corals, is the 

 stratigraphic equivalent of bed 3 of the preceding section. This bed dips a 

 few degrees east of north at an angle of about 4°. The fossiliferous bed is 

 overlain by about 3 meters of gravel. 



Very nearly the same horizon is represented in the two bluffs on the left 

 bank of Rio Mao between Cercado de Mao and Mao Adentro (stations 

 8528 and 8734). The lowermost of these, which is about 25 meters high 

 and is capped with gravel, is composed of bluish silty clay, bluish silt, and 

 sand. The dip averages 7°, nearly due north. Plate XIII, A, is a view 

 of the bluff at station 8528. 



The entire thickness of the Gurabo formation is probably comprised 

 within the interval between the top of the ridge south of Cercado de Mao, 

 which is capped by the Mao Adentro limestone, and the river at its foot. 

 According to a barometric reading the altitude of this ridge above the river 

 is about 230 meters. The thickness of the Gurabo formation on Rio Mao 

 is probably about 200 meters. 



Excellent exposures of the Gurabo formation were noted on Rio Guana- 

 juma at the crossing of the trail from Potrero to Cercado de Mao. The 

 formation here has the same lithologic character and faunal content as on 

 Rio Mao and Rio Amina. 



Large collections of fossils were obtained from the Gurabo formation at 

 Potrero, on Rio Amina (station 8516). The principal collection was made 

 from a 15-meter bed of bluish silt overlain by river terrace gravel deposits, 

 which are exposed in the right bank of the river from a point just above the 

 ford at Potrero for about 300 meters upstream. The dip of the Gurabo 

 formation in this bluff is about 6° toward the north. 



