68 GEOLOGICAL RECONNAISSANCE OF THE DOMINICAN REPUBLIC. 



Pockets in the blue marl contain a great profusion of mollusks. Aphera 

 islacolonis is regarded by Maury as characteristic of the Cercado formation. 



The fossils that have been identified in the collections from the type 

 locality of the Cercado formation (station 8525) are listed on pages 115-126. 



According to information received from Maury, her Bluff No. 2 is 

 the low bluff on the left side of the river between Paso del Perro and Paso 

 Chorrera (station 8526; see sketch map, PL IX). The following is a sec- 

 tion by T. W. Vaughan of the exposure there : 



Section at station 8626, on Rio Mao. 



Meters. 



Gravel, sand, and silt of a stream terrace deposit, unconformable 



on the underlying Miocene 3 to 6 



Miocene (Cercado formation): 



1. Bluish fine silty sand, mostly fine-grained, with pockets and 



layers containing many mollusks as well as Foraminifera, 

 which are in places so abundant as to produce a forami- 

 niferal sandstone. The individual beds range in thick- 

 ness from 0.3 to 1.5 meters and include hard layers 6 to 9 



Total thickness of exposure 12 



The beds dip northward at an angle of 4°. A list of the few fossils col- 

 lected in the lower bed at this locality is given on pages 115-126. 



The Cercado formation is exposed on Rio Cana at the ford of the trail 

 from Las Caobas to Sabaneta, near the villages of Cana (east bank) and 

 Caimito (west bank). The section below is given by T. W. Vaughan : 



Section on Rio Cana at crossing of road from Las Caobas to Sabaneta. 

 Soil and river gravel at top. 

 Miocene (Cercado formation): Meters. 



6. Sand, yellowish on weathered surface 2.1 



5. Conglomerate and coarse sand 1-5 



4. Greenish sandstone, some large grains as much as 6 mm. 



in diameter; a seam of pebbles 4.9 



3. Bed of small pebbles; pebbles mostly less than 25 mm. 



long 0.3 



2. Bluish sands; some pebbles. The sandstone weathers to 



yellowish sands. Contains many fossils in pockets.... 5.3 

 1. Dark-bluish sands, compact and somewhat indurated, 

 interbedded with conglomera+e. The lowest 1 meter 

 is mostly sand with a little clay, and it contains a 

 pebble seam a few centimeters thick. This is over- 

 lain by a pebble bed about 1.3 m. thick. The pebbles 

 range in length from 3 to 15 centimeters. The edges 

 of most of them are rounded, but many are sub- 

 angular. Composed of igneous rock, laminated 

 hornstone, etc 3 . 7 



From bed 2 a considerable number of fossils were collected (station 8534 ; 

 for list see pp. 115-124). 



