. 
112 J. D. Dana—Results of the Karth’s Contraction. 
tains on the principle of UMguation—the feldspathic material 
being the least fusible, and being therefore left at the center 
while the more fusible iron-bearing lavas were drawn off by 
the outbreaks through the different sides of the cone. The 
scale over the Pacific slope of the Rocky Mountains. The 
trachytic rocks there appear, as stated by Richthofen,* to be 
generally the older. Clarence King, in his description of the 
Shoshone Falls on Snake River, states that out of seven hun- 
dred feet in thickness of igneous rocks exposed in the bluff, the 
lower 300 feet are made up of trachyte, while the upper 400 
are of basalt; and that a continuous field of igneous rocks, 
mainly basaltic at surface, stretches over the country of Snake 
River for three hundred miles or more. The outflows are of 
later date than the Miocene Tertiary. The fact of the very 
wide geographical distribution of the basalt on the Pacific . 
slope appears to be good proof, as in the case of the dolerite of 
the Atlantic border,—but better because of the wider range of 
the ejections,—that the source of its material was not local, or 
dependent on the fusion of sedimentary strata. And if this 
be true of the basaltic rock, it is probably so also of the tra- 
chyte. Whatever doubt may exist, the general argument is made 
a demonstration by the fact explained,+ that a vast undercrust 
fire-sea was a necessity in order that the great heayings an 
bendings of the earth’s crust essential to mountain-making 00 
the Pacific border should have taken place. 
The conclusion arrived at militates not only with the theory 
of Hunt, but also to some extent with Mallet’s, unless the 
latter is made to appeal to the true crust for the material to be 
fused by the motion attending mountain-making. The motion 
in the true crust, even in the catastrophic period of mountain- 
making, is very much less and slower than that which is expe 
rienced by the plicating strata above it, and must therefore be 
a much feebler source of heat and fusion. Still, under the frac 
tures, and shovings, and crushings, which must at times take 
place, it should be sufficient; and, acting at infra-Archean 
depths, it would give uniform results over wide areas, or 00 
the other hand a degree of diversity. Unlike Hunt’s hypo 
thesis, Mallet gives a reasonable source for the heat occasion 
ing fusion. 
_But the sufficiency of the method for all cases of igneous 
ejections may well be questioned. The subsidence ending i0 
* The Natural System of Voleanic Rock if i., Vol. I, Part 
2, 4to, San Pesdcinses 1860. The trach st gt olen se ei aa than 
the dolerites of the region. But more recent trachytes occur in Italy, and 
rock of | eruptions in Iceland is trachyte. 
+ This volume, page 7. 
