38 EXPLORING EXPEDITION FROM SANTA Ffi 



The relations which tlio Santa Fe Mountains sustain to the geology of the sur- 

 rounding region may be given in a few words. Like most of the ranges which go to 

 make up the complex group of the Rocky Mountains, though attaining a greal altitude, 



this is comparatively short. How far it may be traced to the north, we are at present 

 not able to say; probably not more than lilt)' or sixty miles. It is then succeeded by 

 other ranges, which, with more or less interruption, extend to and beyond the parks, 

 forming the broad mountain belt which gives character to the topography ot the cen- 

 tra] portions of the continent. On the south it falls off abruptly near Santa Fe, and 

 either wholly disappears or is represented at a distant point by some of the mountain 

 chains which lie east of the Rio Grande. The trend of this chain — a few degrees west 

 of north and east of south — would, if prolonged, strike altogether to the eastward ot 

 its nearest neighbors on the south, Placer and Sandia Mountains. The altitude of its 

 highest peak is supposed to be between twelve and thirteen thousand feet. 



Upon the Hanks of the Santa Fe Mountains, where not removed, the sedimentary 

 rocks rest in regular order, and are, with the exception of the Tertiary, nearly or 



quite conformable. 



THE PLACER MOUNTAINS. 



From the southern base of the Santa Fe Mountains the surface descends toward 

 the south, to the valley of (Jalisteo Creek, some twenty miles distant : thence it rises 

 again, three or four miles, to the base of the Placer Mountains; the course of (Jalisteo 

 ('reek being nearly east and west, following the line of greatest depression between 

 the adjacent mountain groups. 'Phis depression forms a natural pass from the- plains 

 to the valley of the b'io Grande, affording a practicable and convenient railroad route. 

 Viewed as a whole, the interval between the; Santa Fe and Placer Mountains may be 

 regarded as a plain, the inequalities of the surface being slighl compared with those 

 of the surrounding country. It is cut, however, by numerous eroded valleys and 



ravines, and a limited portion of it is occupied by a group of erupted hills — Los Cer- 



rillos — some of which have an altitude of several hundred feet. 



The rocks exposed in the valley of (Jalisteo Creek are mostly of a sedimentary 



character, representing portions of the Triassic, Cretaceous, and Tertiary formations. 

 K.ist of the Cerrillos, though much disturbed, these are but little changed in structure; 



the Cretaceous strata abounding in fossils characteristic of the middle and lower beds 



— the first, mollusks; the second, angiospermous leaves — the Triassic rocks containing 



-reat numbers of silicihed trunks of coniferous trees similar to those so frequently 

 met with in the same formation farther westward. The stratified rocks are. however 

 in several localities, cut by trap dikes, which, offering greater resistance to erosion, 

 have been left in Btrong relief above the surface. Two of these, one north, the other 

 south of the village of (Jalisteo, are particularly conspicuous, running with a nearly 

 parallel course in an east and west direction for several miles across the country. Of 

 these the southern is the most remarkable, standing like a huge, nearly-continuous 

 wall, its crest being in many places a hundred and fifty feet above the plain bordering 

 it on either side. West of the Cerrillos similar but smaller dikes are common, and a 

 large area on both sides of the Rio Grande is nothing less than a lava-plain, the Hood 

 which formed it having apparently flowed from one or several volcanic vents near the 



