CHAPTER IT. 
Geology of the Mountain Ranges. 
Ranges probably granitic -with Carboniferous limestone.—Organ mountains.—Hueco mountains.—Los Cornudos.—Polished 
rocks.— Guadalupe mountains.—Trend towards the Witchita mountains.—Sierra de los Alamos and Los Cornudos.— 
Probable shallowness of the valleys in the granite.—Natural tanks ot reservoirs of water.—Similar tanks in the African , 
deserts.—Probable origin. 
I do not find many specimens from the mountain ranges in the collection, and their geologi¬ 
cal characters along the route are, therefore, not very fully represented. These ranges are the 
southern continuations of those hordering the valley of the Eio Grande, opposite Albuquerque 
and Santa Fe, and which are known to he composed chiefly of granite and gneiss, with overlying 
Carboniferous limestone hearing the usual fossils. The observations of the gentlemen connected 
with the United States and Mexican boundary commission at the south of the Guadalupe Pass, 
have shown that the southern prolongation of the chain, from which the Guadalupe range 
diverges, is composed chiefly of granitic rocks; the presence of Carboniferous limestone and 
eruptive rocks has also been shown at the southern end of the Organ mountains, near El Paso 
and Fort Bliss. 
Organ mountains. —I do not find many recorded observations upon the rocks which compose 
the Organ mountains, or the Sierra de los Organos —so called, it is said, from the fancied resem¬ 
blance of the highest peaks to the pipes of an organ. Captain Marcy states that they are of 
Trap formation, and somewhat columnar in structure, with the columns standing vertically, and 
in some cases rising to the height of a thousand feet, and terminating in sharp points. 1 
The southern part of this range is, however, known to he granitic, and to he partly composed 
of Carboniferous limestone, which, perhaps, presents mural faces; and these, seen from a distance, 
might he mistaken for bluffs of trappean rock. There are two small specimens of a dark-blue 
limestone from the Organ mountains in the collection, and they are evidently fossiliferous. 
They resemble specimens of the Carboniferous or Mountain limestone, to which formation they 
may he safely referred. 
Hueco mountains. —The specimens which Captain Pope brought in from the Hueco mountains 
show that they are composed of granite; and if the great mass of rock is like the specimens, it is 
fine-grained and compact, and probably eruptive, not formed by the alteration of stratified rocks. 
It is, however, possible, and very probable, that the rocks usually called metamorjohic are 
present in that range, and the presence of a small mass of white crystalline limestone with the 
specimens may he regarded as an indication of them. In the Diary of Mr. Byrne, I find it 
stated that the Hueco mountains at the tanks “are composed of a dark-grey sandstone scattered 
about in high masses, in the most grotesque disorder and confusion.” 2 
I find very nearly the same observation made in the report of Captain Marcy, who traversed 
the range in 1849. He writes : “ The road passes between the two mountains, which approach 
within a few rods of each other, leaving a level pass, bordered by immense ledges of rocks 
standing out in hold relief directly over the road. The rocks composing the mountains are 
1 Report on the Route from Fort Smith to Santa Fd, p. 198. 
* Report of Captain Pope, Appendix A—Diary of J. H. Byrne, p. 53. 
