IgE 
GEOLOGY. / 101 
The various proportions and state of aggregation of these simple minerals, and differences in 
external forms, give rise to varieties which have received distinct appellations, such as basalt, 
amygdaloid, dolerite, greenstone, and others. The term is derived from trappa, a Swedish 
word for stairs, &c., &c." — Lyell: Principles of Geology. | 
More westward the mica in this rock becomes scarce, though it never disappears entirely from 
it. Near camp 53, where we left the Sahwatch creek in order to go over the pass, the mountains 
on the right bank of the little creek, coming down from the north, are composed of a kind of 
granitic rock, black mica imbedded in a crystalline mass of glassy feldspar. The only sedimen- 
tary rock found between Sangre de Cristo valley and Coochetopa creek was a hard, fine-grained 
sandstone near the divide of the Sierra St. Juan. 
n the west side of the Coochetopa Pass, we meet again with two kinds of porphyry; the 
one resembling the gray porphyry of Sangre de Cristo, the other that of the Sahwatch butte, 
containing, however, more mica, In the lower regions of the mountains, forming the valley of 
the Coochetopa creek and the Grand River valley, feldspathic granite, gneiss, a rough silicious 
shale and a fine mica slate, the latter dissolving only under a powerful lens into a mixture of 
quartz and mica, and a white sandstone, are the predominating rocks; in the upper regions, 
sandstone and trapp-porphyry, the latter resembling very much the trapp-porphyry of Sahwatch 
valley. It consists of a compact, hard, brown-violet feldspathic base, interspersed with crys- 
tallized glassy feldspar, and a few mica crystals. Thin splinters of this base melt more easily 
on the edges before the blow-pipe than the Sahwatch porphyry. This rock lies mostly hori- 
zontal, about sixty to seventy feet thick, on the summit of the mountains, over a white sand- 
stone, forming sometimes remarkable platforms. It seems to have an immense extension, for 
we found it for over a hundred miles along our road. As it must necessarily have been in a 
state of fusion at the time it spread over the sandstone, we may form an idea as to the scale on 
which eruptions of igneous rocks have taken place at a time when sedimentary strata of consid- 
erable thickness were already in existence. 
By contact with granite, this rock has in some places (along Grand river) become blackish 
and very brittle. It seems to have been displaced when already in a solid state, for it has 
polished surfaces which could only be produced by its sliding over some other solid rock. I 
cannot positively assert that this rock is an altered trapp-porphyry, for I could nowhere trace a 
direct connexion; but in their lithological character, they stand very near each other. The 
narrow cañon of Grand river, below camp 58, is formed of granite and this rock. Some 
miles above that cañon, a conglomerate of igneous rocks of all kinds, imbedded in a very hard 
cement, lies under the sandstone; and when, during and after the gradual upheaval of the 
mountains, the other rocks were completely washed and worn away, this conglomerate partially 
resisted the action of time and the weather, so that parts of it are seen hanging over the steep 
sides of the mountains in the shape of towers, resembling very much those ruins of old castles 
which, though remnants of barbarous ages, give unequalled charms to so many landscapes in the 
eastern world. The great hardness of the cement of this conglomerate—I nearly broke my steel 
hammer in the attempt to break some pieces off—induced me to make some chemical experi- 
ments as to its composition. It consists, however, of nothing but impure carbonate of lime, 
being entirely soluble in diluted chlorhydric acid. The solution evaporated to dryness, and 
re-dissolved, does not leave a trace of silica behind. 
The country between the St. Juan and Wahsatch mountains is a barren, dreary desert. The 
road leads mostly over sand or its generator, sandstone; which latter constitutes the several 
smaller mountain ranges between these two great chains. Sandstone, assuming all shades 
of color, sandy calcareous clay slate, argillaceous limestone of green and red colors, sandy shale, 
and, uppermost, a soft foliating shale from gray to black, including much fibrous and lamellar 
gypsum, seem to be the formations composing the «Elk mountains, All these different 
strata are concordant, and of a slight northeast dip. Some miles distant from where we crossed 
