84 



ON THE TOPOGKAPIIY AND fiEOI.OGY 



lie in a series of east and west folds, the line of folding- and npheaval corresponding very 

 closely with the axis of the mountains. The thickness of the deposit is very dithcult 

 to determine, since no continuous section exists, where one can be certain of having 

 all the beds, and of not being deceived by rcj^etitions. On a very rough estimate, 

 not based on measurements, however, but only on the broadest kind of vague generali- 

 zation, in the caiions of the Ocoa and ISTigua 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 l)e understood that this l>road 

 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 con- 

 glomerates occurs ; in another, within forty or fifty miles these same beds are repre- 

 sented by limestone, without a pebble, and on the same strike within another tAventy 

 miles, neither limestone nor conglomerate is to be found ; all is a semi-talcose slate too 

 friable to yield a hand specimen, and with no distinguishable 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. 



In the interior of the mountains, especially in the western two-thirds, the disturb- 

 ance has lieen greatest, and the reason appears, in the existence of great masses of 

 eruptive rock which have pushed up the slates, broken them, and in some cases pene- 

 trated them by dykes to a distance of several miles from the ])arent mass. In some 

 cases pieces of the wall rock are found inbedded in the syenite ; and pebbles one-half 

 syenite the other jaspery slate are not rare. 



In its original state, this group of beds seems to have consisted of a scries of clay 

 shales, thinly l)edded, others more heavily bedded and Avith layers of sandstone, con- 

 glomerate, limestone, and heavy bedded sandstones. The changes produced in these 

 rocks by metamorphism are almost infinite. On the Ocoa River the shales are so 

 nearly unaltered that I have repeatedly searched in this region in hopes of finding 

 fossils. In the caiion of the river they are gray and friable, with an occasional bed of 

 sandstone ; further south they are red, and give rise to numerous salt springs ; still 

 further south they contain more numerous beds of sandstone, and are brown and more 

 sandy in texture. These same shales are modified at Recol into a granular, greenish- 

 black material resembling an impure serpertine,'^" while on the i^'igua they appear as 

 green, gray or brown jaspers with broad conchoidal fractures, gradually changing 



* A similar change occui s on the north tiank of Monte Diablo, California, where I have followed the same bed of 

 rook along a series of outcrops ; in one place, simp'e unaltered sandy shale, giadually modifying until it became in 

 another a nearly pure serpertine. See WJiitnei/, Geol. Beport Cit.l., vol i. p. 32. 



