W. M. Gabh on the Geology of Santa Domingo. 253 

 the geological features which are intimately connected with 



The great central chain of mountains consists of an immense 

 cone of syenite and syenitic rocks, evidently of later date than 

 the metamorphic strata that flank it. This mass is probably, 

 in some places, as much as fifteen or twenty miles wide, though 

 its southern and southwestern borders have not yet been ex- 

 plored, lying as they do in the Province of Azua, to which 

 our labors have not yet extended. In this Eepublic it makes 

 its appearance on the borders of Hayti, about fifteen miles 

 south of Manzanilla bay, and from here makes all of the higher 

 range of central mountains, to a point just about due north- 

 west of Sta. Domingo City, or in other words, the center of the 

 Republic ; there its northern boundary suddenly bends south, 

 becoming the eastern, the mass making a tongue eighteen to 

 twenty miles wide, running a little east of south, to a point 

 about twenty miles from the coast and nearly north of Bani. 

 From analogical reasoning, based on the character of its north- 

 ern margin and the peculiarities of the topography, it is almost 

 safe to predict, that north of Banica and San Juan, or in other 

 words in the mountains of the north-west, it will not have a 

 much greater width than the strip above Bani. This mass of 

 crystalline rocks has pushed up, tilted, folded, and in some 

 places sent complicated net works of dikes into the overlying 

 strata. Some of the dikes extend miles from the parent mass, 

 and are of all sizes from 100 feet and upward, down to a thread. 



The lithological characters of the syenites are not so variable 

 a« might be anticipated over so wide an area (say 20 by 100 

 mfies). The rock is usually a light gray, moderately fine- 

 grained mixture of the ordinary constituents, quartz, feldspar 

 and hornblende, in nearly equal proportions, though sometimes 

 ine latter mineral makes up almost the entire mass, in the shape 

 01 large crystals, and more rarely it is almost entirely absent, 

 and still more rarely a little mica occurs. No gneiss has been 

 obseired, but two or three localities of mica slate have been 

 tound, and one or two erratic pebbles have been discovered 

 composed of quartz and feldspar only. . . 



Uverlying the syenite, and as stated above, uptilted by it is 

 a tieavy deposit, several thousand feet thick, of conglomerates, 

 jaspery and magnesian slates, with a httle lunestone. These 

 are almost invariably metamorphosed to such an extent as to 

 j;ave entirely destroyed their original character, and often, even 

 jeir stratification. In the country west of Bani, both about 

 r/^wer Ocoa and farther north, the shales are so little altered 

 ^at they can be recognized as fissile clay shales, in a few ca^es 

 f^^ nse to salt springs, but in no case fossiliferous. I was 

 Z?iT^ enough, at one locality on the Nigua, 1« discover a 

 ««iall locality in the Hmestone, almost unaltered and with a few 



