642 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



and volcanic extrusion, and a conflict between disturbed and quiescent conditions of deposition 

 wMch. finally culminated in the establishment of the latter in the succeeding Richmond epoch. 



For a description of the Richmond and later beds of the early Tertiary, see 

 Chapter XVI (pp. 717-721). 



E 19. SANTO DOMINGO. 



The Sierra group of Santo Domingo was described by Gabb as of Cretaceous 

 age. His account of the rocks shows them to be highly metamorphosed and varied 

 in character, and generally unfossiliferous. For this reason the major part of 

 the terrane is mapped as Paleozoic undivided, and his description is quoted in 

 Chapter VII (pp. 345-347). The description of the limestone which contains 

 Cretaceous fossils is as follows : ^^^* 



On the Nigua River the limestone acquires its greatest development. Here the metamor- 

 phic action has been unusually well marked and the formation is represented mainly by hme- 

 stones and jaspers. The stratification also is unusually well preserved, so that a good section 

 is attainable. The lime strata of the Ocoa are apparently aU repeated on the Nigua, and -the 

 conglomerates are replaced by beds containing but few pebbles, the matrix being either a pure 

 or more usually an earthy limestone. This latter rock, at two localities on the river, yields 

 fossils, in a bad state of preservation for extraction, although occasionally recognizable. The 

 shell substance is completely crystallized, so that it is next to impossible to extract a thick 

 shell in such a manner as to expose its surface. Of hundreds of attempts to obtain a speci- 

 men of a very common Trigonia I have only succeeded once or twice in obtaining a little piece 

 of the surface; the fracture always taking place ainong the crystals of calc spar into which 

 the shell has been cemented. The univalve shells invariably break across and, in a word, 

 the collector has to content himself with the few imperfect fragments found weathered out. 

 Nowhere else have fossil MoUusca been discovered in this formation in Santo Domingo, and 

 only in one other instance have I found any other fossil in it. Three or four miles west of 

 San Jose de las Matas I was fortunate enough to discover two fragments of the same Umestone, 

 being of a dark bluish-gray color, on the surface of which can be detected, faintly marked, 

 the stars of a coral. Doubtless by means of polishing they can be brought out. These are 

 of especial value, being the only traces of corals yielded to us by the formation; unless indeed 

 a stray pebble, found on the surface of the ground near Bani, also containing corals, may 

 ■ belong to the same group. On the peninsula of Samana the limestones are highly metamor- 

 phosed, and here occurs a very curious mixture. In some cases the limestone has a few 

 scattered scales of mica embedded in it; in others the mica is so abundant as to form layers, 

 while not infrequently, especially in the eastern part, mica slate, alternating with the lime- 

 stone, is not an unusual feature. This is the more remarkable since it occurs nowhere else 

 on the island, and in only one other locality is mica found, and there in hardly noticeable 

 quantities. ' 



With so few data it would seem hazardous to venture a determination of the geological 

 age of the group of rocks in question. But a careful study of the scanty material at my dis- 

 posal reveals the existence of a serrated oyster, a Trigonia, a TurriteUa, shells resembling 

 AnciUaria, Natica, PugneUus (?), and Mactra, besides a fusoid, one fragment that seems to 

 belong to the group of Pterocrea — -at least it is an alate shell — bivalves that may prove to 

 be CucuUsea and Lima, and better than all else a beautiful httle ammonite in perfect preservation, 

 and a fragment that I think more careful study will decide to be a piece of a baculite. From 

 the above list it wiQ be seen that there can be no possible doubt of the secondary age of the 

 rocks. The ammonite restricts it so far. The style of the ammonite besides confines it to 

 the two groups of Jurassic and Cretaceous, a determination corroborated by the Trigonia, 

 which further belongs to a type much more common iii the latter than in the former formation, 

 resembling T. emoryi Con., T. evansaTia Meek, and T. mooraena Nob. (T. crenulata Roem., 



