802 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



along both sides of Samana Bay. In the south it forms a little group of hills extending from 

 near the Jaina River across the Nigua and thins out in a few isolated patches toward the Nizao. 

 Its total thickness in the vicinity of the Nigua River is about 400 feet, and it is made up of a 

 succession of brown earthy and sandy beds, occasionally calcareous, superposed on a thin stratum 

 of conglomerate. The top of the series is a rather compact calcareous deposit containing 

 corals. The fossils, except the corals, are usually badly preserved and very meager in species; 

 a small oyster, two species of Pecten, and some echinoderms being the only recognizable forms. 

 Besides these, a few internal casts of gastropod shells have been discovered. The corals are so 

 embedded in their matrix that they can only be collected satisfactorily when they are weathered 

 out, and this same process of weathering is only too apt to destroy the delicate structure of 

 their surfaces. They are, however, nearly all of the massive forms and are so thoroughly 

 fossilized as to be well adapted for polishing. 



But while the formation is so small in area and so unsatisfactory in general characters on 

 the Santo Domingo side, it becomes, in the Cibao, the most interesting on the island. Cut 

 through by all the tributaries of the Yaqui the sections are numerous and excellent for study. 

 Its local modifications are well illustrated by sections innumerable, into and across the Monte 

 Cristi Range on one side and into the southern hills on the other. In short, it would be difficult 

 to find a region where the facilities furnished to the geologist are greater, or where the results, 

 could be more certainly arrived at. Add to this that a larger part of the formation is highly 

 fossiliferous, and that the fossils, whether shells or corals, are almost always preserved entire 

 and hard; as beautiful as the famous fossils of the Paris basin, or as the less known though 

 equally beautiful specimens from Jackson, Miss. 



The entire thickness of the formation in the Cibao is probably over 1,500 and under 2,000 

 feet. It is made up of coarse sandstones at the base, sometimes bearing beds of conglomerate, 

 which are, however, rather local in extent. These beds are best developed between the Bao 

 and the Yaqui, where, being uptilted, their thickness is best seen, and are about 600 feet thick. 

 They gradually merge into gray shales, which form a transition to the heavy blue-shale beds 

 underlying the town of Santiago and called by the English paleontologists the Nivaje shale. 

 The upper part of this member is always of a light yellowish brown or buff color and some- 

 times, especially at its upper part, contains beds of sandstone. The whole of this shale mem- 

 ber may be safely estimated at about 800 feet of average thickness. The remainder of the 

 formation, say 400 feet more in all, varies locally. It caps the greater part of the Monte Cristi 

 Range, and while north of Moca it forms high bluffs of a nearly white earthy rock, in which it is 

 doubtful whether the argillaceous or the calcareous ingredients preponderate, it forms north 

 of Esperanza sheets of a compact limestone, which, less pure, forms the cap on the isolated 

 table mountain of Monte Cristi. Near Cevico it appears as an impure lime rock containing 

 corals and Foraminifera, and similar beds occur also on Samana and south of Savana la Mar; 

 while the caves of San Lorenzo, in the same beds, are in a hard, coarse-grained calcareous sand- 

 stone. Nor do the variations cease here. Near the mouths of the ancient Miocene rivers run- 

 ning from the then much smaller islands now constituting the Cibao Mountains, and among 

 which the Mao was probably the largest, the gravel debris of these streams was deposited, 

 occasionally alternating with a bed of coarse sandstone, synchronously with the formation 

 of the coral reefs and beds of fine white mud which now glistens in the sun on the top of the 

 Monte Cristi Range. We have thus an ideal section as follows: 



Feet. 



White calcareous marl, north of Moca; white or light-brown limestone ("Tufaceous limestone" 

 of Heneken) ; light-brown calc sandstone, San Lorenzo Bay; gravels of Mao and Savaneta; lime- 

 stones of Samana, San le Mar, Cevico, and the north face of the Samba Hills; oyster beds of 

 Samba Hills, south of Guayubiu 400 



Brownish or yellowish shale of Guayubin; conglomerate of Angostura of the Yaqui; sandstone 

 strata near Santiago; dark-blue shale of Santiago; gray shale with beds of sandstone of Bio 

 Verde and in the hills north of Moca 800 



Coarse gray sandstone with some conglomerate; seen best in the hills south and southeast of San- 

 tiago, also in a lew places in the north range , 600 



