346 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



Cretaceous and Tertiary age, made up of igneous rocks and clays, mantled by gravels and crys- 

 talline limestone; the white limestones, of Tertiary age; recent alluvial formations; and the coast 

 limestone of elevated reef rock. No recent volcanic rocks are known. The geology and minerals 

 of Santo Domingo have been the subject of special reports by many writers, including three 

 American geologists, Messrs. Blake, Gabb, and Marvin. 



The terrane which is mapped as undivided. Paleozoic on the island of Santo 

 Domingo corresponds to the Sierra group of Gabb, who placed it in the Cretaceous. 

 According to his description he collected distinct Cretaceous fossils in the southern 

 part of the island, but elsewhere the strata are metamorphosed to such an extent that 

 no fossils have been found. In view of the facts that much of the terrane is highly- 

 metamorphosed and that the discovery of fossils has been limited to the southern 

 margin of the uplands, it seems not unreasonable to suppose' that some of the rocks 

 may be not only oldei than Cretaceous but possibly pre-Mesozoic. Were they 

 colored as Cretaceous they would naturally be associated with the fossiliferous 

 Cretaceous limestone of the West Indies and the adjoining continent. The com- 

 pilers have therefore distinguished them by placing them in the somewhat indistinct 

 class of undivided Paleozoic rocks, including areas of younger strata. That portion 

 of the strata in which Cretaceous fossils have been found is mapped as Cretaceous. 

 Gabb ^^^ says : 



No formation older than the secondary era has been found on the island, the oldest group 

 being the great mass of slates, conglomerates, and limestones which form its core. These are 

 uptnted and broken by numerous intrusive masses of crystalline rocks which may be, for con- 

 venience, grouped under the generic term of syenite, since they almost invariably consist of the 

 three necessary minerals quartz, feldspar, and hornblende. 



Flanking the slates, etc., of the Sierra there is a broad development of Tertiary marking 

 all the northern and a part of the southern side of the island, and this in turn is bordered by a 

 more recent deposit of limestones and gravels which I shall call the Coast formation. 



The Sierra group forms all of the great mountain mass of the interior, extending the entire 

 length of the Republic. 



It also constitutes the greater part of the Peninsula of Samana and appears as a single 

 little outher, under the Tertiary, near Puerto Plata. It everywhere shows the evidence of 

 active subterranean forces, being not only metamorphosed, with hardly a single local exception, 

 but is everywhere much uptilted and usually strongly folded. Over much of its area the meta- 

 morphic action has been so complete as to destroy the traces of stratification or to so nearly 

 obliterate them that they are apt to be confounded with cross fractures. * * * 



But enough of the stratification is preserved to show conclusively that these beds lie in a 

 series of east and west folds, the line of folding and upheaval corresponding very closely with 

 the axis of the mountains. The thickness of the deposit is very difficult to determine, since 

 no continuous section exists, where one can be certain of having all the beds and of not being 

 deceived by repetitions. On a very rough estimate, not based on measurements, however, but 

 only on the broadest kind of vague generalization in the canyons of the Ocoa and Nigua rivers 

 and again on what I saw of the formation in the vicinity of the Pico del Gallo, we might set 

 down the total thickness at anywhere between 2,000 and 4,000 feet. It must be understood 

 that this broad margin is not the result of want of care in observation but arises from the almost 

 absolute impossibility of finding a reliable section. In one region a group of conglomerates 

 occurs; in another, within 40 or 50 mUes, these same beds are represented by limestone, without 

 a pebble; and on the same strike within another 20 miles neither limestone nor conglomerate 

 is to be found; all is a semitalcose slate too friable to yield a hand specimen and with no distin- 

 guishable stratification. Add to this the partial obliteration of character by different degrees 

 of metamorphism, and the sometimes total obliteration of bedding, and the reasons of my cautious 

 statement must be sufficiently obvious. 



