GEOLOGY AND GEOLOGIC HISTORY. 71 



of Goniopora, Orbicella, and Siderastraea nearly 1 meter in diameter are not 

 rare. The species of fossils that have been identified in collections from the 

 Gurabo formation along Rio Gurabo are listed on pages 133-147. The 

 locations of the stations at which they were taken are shown by num- 

 bers corresponding to those on Plate XI. 



The Gurabo formation appears to be conformable with the under- 

 lying Cercado formation. As the lower part of the Mao Adentro limestone, 

 which overlies the Gurabo formation, contains local deposits of con- 

 glomerate, it seems probable that crustal movements were going on at the 

 close of Gurabo time, but deposition appears to have been continuous. 



Along Rio Mao the Gurabo formation is exposed in every bluff from a 

 point about 5 kilometers south of Cercado de Mao to the first bluff above 

 Mao Adentro, about 3 kilometers north of Cercado de Mao. (See PL IX.) 

 The beds are of the same lithologic character here as on Rio Gurabo, and 

 they contain the same fauna. The formation dips generally northward 

 at a low angle, probably in most places less than 5°. 



The following section was measured in the first bluff on the east bank of 

 Rio Mao above the ford known as Paso la Chorrera: 



Section on right bank Rio Mao above Paso la Chorrera. 

 Miocene (Yaque group): Meters. 



Gurabo formation: 

 6. Inaccessible. 



5. Yellowish calcareous sand and hard nodular lime- 

 stone containing Placocyathus and casts of 



mollusks 3 



4. Concealed. About 9 



3. Fine, friable yellowish sand; when fresh probably like 



the underlying beds. About 32 



Cercado formation: 



2. Compact, fine-grained calcareous sand, with ledges of 

 nodular limestone. Contains a bed rich in 

 shells, overlain by a layer consisting chiefly of 

 Foraminifera which, where locally indurated, 



resembles a coarse sandstone. Station 8522 4.5 



1. Compact, fine-grained calcareous sand with ledges of 

 nodular limestone. Contains mollusks, corals, 

 and imperfect plant remains. Certain layers 

 contain great numbers of small Foraminifera. 

 Station 8521 4 . 5 



The first bluff below Paso la Chorrera (see PI. XII, B), which is also the 

 first bluff on the west side of Rio Mao above Cercado de Mao, furnishes a 

 good section of the Gurabo formation. This bluff, which is called "Bluff 

 1, Cercado" by Maury, 1 is one of the localities she cites 2 as typical of 

 the Gurabo formation. The lower bed of this section contains the fauna 

 of the "Sconsia laevigata zone." 



1 Maury, C. J., Am. Paleont. Bull., vol. 5, pp. 425-427, 1917. 

 ! Maury, C. J., Science, new ser., vol. 50, p. 591, 1919. 



