1008 Super-Metamorphism and Vulcanism. [December, 
the lavas of the Western United Statés is, in the rough; of such 
wide application that we must expect to discover more thana 
mere accidental cause. The occurrence of onlyone type of lava, 
as andesite, trachyte, rhyolite, or. basalt, may be readily explained 
as due to the overcoming of the resistance to outflow at one or 
other stage of the process of fusion. So, in certain wide areas, 
it might be possible for all of Richthofen’s types to be ejected 
from as many distinct orographic centers. But in Wyoming and 
Colorado two great districts have the old volcanic vents so re- 
lated to each other, in the several flows, that one can not avoid 
the conclusion that each field ‘has been the seat of one long-con- 
tinued period of activity marked by successive epochs of erup- 
tion. Thirteen years of study in these regions have revealed 
many facts bearing upon these questions. Having elsewhere out- 
lined a plausible theory of vulcanism,! based upon these and gen- 
eral information gleaned from ‘the West Indies in merely travers- 
ing that region, together with the published accounts of leading 
authorities, I shall not here attempt a discussion of it, but confine 
myself to a simple statement of its main points. 
In brief, then, it seems evident that the earliest volcanic out- 
flows came out through lines of least resistance in the axes of 
folds in the strata. In cases where these lines coincide with the 
major folds and the lines of maximum tension, the outflow will 
be andesitic or basaltic, č. e., basic. If the tension be not suffi- 
cient to overcome the resistance, more acidic material will be 
formed at the top of the magma, under the folds, and this may 
burst forth as trachyte, or finally as rhyolite, provided that the 
resistance is not sooner overcome. Basalt comes last as the deep- 
seated, heavier portion of the magma, and in some cases this fol- 
lows andesite without the intervening trachyte and rhyolite. 
In the San Juan mining region and in the Yellowstone Park 
area, the necessary conditions for the successive ejections have 
been brought about by a somewhat complicated series of foldings, 
-eross-foldings and faults, accompanied by an elastic crust of sili- 
a  ceous material. The subject is one which can be studied in these 
a regions to great advantage, but we are only beginning to under- 
stand how simple is the problem which nature has solved with 
much variety of detail to suit changirig conditions of environment. 
The Geology. and Vein-structure of Southwestern Colorado. By Theo. B. Com- 
nsactions Institute of Mining Engineer-, Bethlehem meeting, Mr = 
ms a og et A 
i 
: 
: 
. 
1 
| 
d 
