GEOLOGY. 
105 
Leaving the canon wherea conglomerate of igneous rocks crops out from under the above- 
described shaly trapp, we no longer meet with any older plutonics, or with any sedimentary 
rocks. Even sandstone, so abundant in the country we before traversed, has disappeared, and 
the more recent rocks, basalt, phonolithe, ringing beautifully to the hammer, and chiefly tra¬ 
chytes, with a number of intermediate rocks, become masters of the ground. The trachytic 
rocks seem to be the predominating, if not the only rocks in those parts of the Sierra which we 
had to traverse when travelling from the headwaters of the Sacramento river down to Honey 
Lake valley, and which are covered with the most magnificent pine forests. 
The highly interesting scenery around the so-called “Black Butte” bears, of all the parts of 
the Sierra Nevada we passed over, most strikingly the character of a volcanic country. Sur¬ 
rounded by elevated peaks and high cliffs, and quite isolated, stands a mountain, from eight 
hundred to a thousand feet high, of conical shape, and formed of black lava, apparently a 
monument of the latest disturbing forces in these regions. The lava is in some degree decom¬ 
posed at the surface, and the butte, as well as the soil around, is covered with volcanic sand and 
blocks or small pieces of that lava. In the cliffs on the west side of the butte, the lava passes 
gradually into trachyte. The summit of this mountain butte is rounded, and no opening in 
it is perceptible from below. 
On the western slopes of .the Sierra we find a granitic trachyte, which is distinguished from 
all other trachytic rocks of the Sierra we met with. It is a reddish gray rough rock, inter¬ 
spersed with crystals of mica, pyroxene—a crystallized mineral, splinters of which melt before 
the blow-pipe into a pearl, giving out an intense light—and another transparent substance, 
on which the blow-pipe has no effect, and which, on closer examination, will very likely prove 
to be a very brittle quartz. The time allowed to this report unfortunately does not admit of 
a thorough chemical examination, as well of this as some other interesting rocks described 
above. It would be of the highest scientific interest to know whether these rocks, and chiefly 
the trachytes of the Sierra Nevada, conform to the law which in the last few years has been 
established by Bunsen. The great importance of this law, which seems to explain complicated 
phenomena in a very simple way, and which is not so generally known as it deserves to be, 
induces me to give a short exposition of it in this report, and to verify it by such analyses as 
I have been able to make during the limited time allowed for the preparation of this report. 
In the fact, established by him experimentally, that the point at which melted substances 
become solid, varies according to the pressure exercised on these substances, Bunsen expected 
to find an explanation for the great variety of eruptive rocks, and for the contradictions in the 
succession of the crystallization of the several mixing substances in relation to their fusibility. 
The analyses of rocks in the lump, without separating the different simple minerals of which 
they are composed, which hitherto have been so much neglected, again become important. The 
analysis of a great number of the characteristic rocks of Iceland, led to the opinion that all these 
rocks have originated from separate or combined eruptions out of only two independent hearths. 
One of these hearths furnished the trachytic, the other one the pyroxenic rocks. Their mixing 
up gave rise to a series of intermediate rocks, which Bunsen called “trachyto-pyroxenic.” 
Great as may be the difference between their ages, mineralogical character, and arrangement, 
the trachytic and the pyroxenic rocks show a constant average composition, which has only in 
a few cases been disturbed by local influences, which are easily traced.* 
The first of the extreme members of these rocks, the normal trachytic, is a mixture of bisili¬ 
cates of alumina and bisilicates of alkalies, the oxygen of the acid to that of the basis being as 
3 : 0.596. In the other extreme member, the normal pyroxenic, the proportion between the oxy¬ 
gen of the acid and that of the bases (alumina, lime, magnesia, potash, soda, protoxyd of 
iron,) is 3 : 1.998, or as 3 : 2, almost. They are to be considered a mixture of bibasic silicates. 
The proportion between silica and magnesia and lime is almost always constant; but that 
between alumina and protoxyd of' iron is subject to great variations, as shown by analysis. 
* Comp. Annual Report by Liebig and Kopp, 1851—1853. 
14 b 
