100 GEOLOGY. 
The chief rock of the Sierra Blanca in and around Roubideau's Pass is an altered, rough, 
bluish violet, mica slate, through which quartzose, granitic, and porphyritic rocks have erupted, 
including pieces of each other, so as to form quite a chaos of rocks. 
The entrance to the beautiful Sahwatch valley is marked by an isolated butte of hard, gray, 
granitic porphyry, with glassy feldspar and mica. The valley is formed by picturesque mount- 
ains, which are steep, often vertical, or of a true trapp form on the south side, less steep and 
covered with timber on the north side, where the destruction of the mountains by the melting 
'snows, rains, &c., is less rapid. The rock which constitutes these mountains is a red trapp- 
porphyry,* with a red feldspathic base, with glassy feldspar and mica. Thin splinters of this 
base melt with difficulty, on the edges, before a well-directed jet of the blow-pipe. 
Limited view of Trapp Rocks, (visible mie e the valley of the ese nin ereek and its branches,) in ascending the 
oochetopa Pass from the 
[* Norg.— The term trapp is used so vaguely, and with so little distinction, that it is often 
impossible to know what kind of rock is meant by that name. With some authors it seems to 
designate any kind of igneous, not granitic, rock. That this repeti of language is a general 
one, will appear from quotations from three distinguished geologis 
** Connected with the aforementioned rocks—diallage rocks, (cimi ;) hypersthene rocks ; 
pyroxene rocks, including basalt, dolerite or greenstone; su phibóné rocks, including diorite or 
greenstone; Grachyüe èni vorphyeite rocks—there is a whole series of rocks which agree with 
those in so far as all of them contain only simple silicates, whilst the granites and their neigh- 
bors, containing an excess of silica, are multi-silicates. But what distinguishes the trappean 
tock from all the preceding ones, is the want of any perceptible structure ; not even the microscope 
shows any structural elements in the trapp rocks. They are in some manner related to the 
basalts; they have the same volcanic origin, were in a state of fusion at the time of their form- 
ation, did separate like those in regular masses, or even in hexaëdrix prisms. Al these rocks, 
which Haüy called * aphanites' on account of their undeterminable structure, are rough to the 
touch. It is probable that chemical analysis will offer the means of classification for these 
rocks; at present this is impossible. In his examination of the trapps of the Ferroe islands, 
Durocher has pointed out the way chemical analysis has to follow in this respect ; he has shown 
these trapps to consist of two varieties, of which one must be reckoned amongst the hyperites, 
the other amongst the euphotides; the latter being easily distinguished by its yielding esl 
when heated." — Elie de Beaumont, in his * Cours de Géologie à l'école des Mines,” German v 
sion, by C. Vogt. 
** Trapp or greenstone is a dark and heavy blackish-green or brownish rock, consisting of 
hornblende and feldspar; it usually has a crystalline texture, but is sometimes compact. When 
albite replaces the feldspar, it is called diorite and diabase. Basalt is a similar rock, &c., &c.”’ 
DE s Mineralogy, (3d edition.) 
. “ Trapp and trappean rocks. Volcanic rocks, composed of feldspar, augite, and hornblende. 
