284 TRANS. OF THE ACAD. OF SCIENCE. 
The rock consists of granite, gneiss, and light-greenish por 
hyry. The granite is hard, compact, and fine-text and 
rocks, the Upper Carboniferous limestone is We meta 
morphosed and inclined at angles of 10° to 30° yond the 
Sierra de los Alamos, this limestone is largely developed, and, 
in places, crowded with characteristic Carboniferous fossils 
It extends to the Sierra Alto, in undulating strata, and the 
surface of the country is marked with low hills and ridges 
from the rocky sides of which fossils were obtained in the 
greatest abundance. 
The Sierra Alto rises majestically from a nearly quadran- 
gular base to the height of about fifteen hundred feet. Itis 
completely isolated from the neighbouring mountains, 4 
rugged summit projects considerably above the est 
them. Its main axis extends nearly east and west, forthe 
distance of about two miles, and its width varies fom 4 
fourth to about three-fourths of a mile. On all sides, it 18 5% 
rounded by hills and ridges, which present abrupt Me 
toward the mountain, and slope gradually in the —— 
direction. The mountain is composed of hard granite . 
light gray color, in which the felspar predominates ove?’ 
quartz. and mica. The weathered face of the rock presents 
highly metamorphosed and of dirty brown and yellow color 
As well as could be determined from their ex ; 
thickness of i 
of the strata becomes much less apparent, 80 that, rr a 
tance of a mile, it does not anywhere exceed 5° to 4s of the 
soon after, we entered an extensive go », whi ae 
the southern side of the mountain. This gorge OF * 4 in 
* up thre 
places, nearly perpendicular esearpments of hard, fine 
gray subcrystalline limestone, and soft, pease anu ple 
grained sandstone, with occasional intercalations ? eragt 
argillaceous shale. Near the eastern extremity: ‘strata are {0 
sae is — a quarter of a mile, and, pote = we advance 
seen dipping southeast from 15° to 25°. ASP or 
air pring the direction 1 of | 
comes much narrower, and 
