m one formation to another; but even here the proof is
generally wanting that the vein materials have been furnished by the
wall rocks opposite the places where they are found.

The varying conductivity of the different strata in relation to heat and
electricity may have been an important factor. Trap dikes frequently
enrich veins where they approach or intersect them, and they have often
been the _primum mobile_ of vein formation, but chiefly, if not only, by
supplying heat, the mainspring of chemical action. The proximity of
heated masses of rock has promoted chemical action in the same way as do
the Bunsen burners or the sand baths in the laboratory; but no case has
yet come under my observation where it was demonstrable that the filling
of a fissure vein had been due to secretion from igneous or sedimentary
wall rocks.

In the Star District of Southern Utah the country rock is Palaeozoic
limestone, and it is cut by so great a number and variety of mineral
veins that from the Harrisburg, a central location, a rifle shot would
reach ten openings, all on as many distinct and different veins (viz.,
the Argus, Little Bilk, Clean Sweep, Mountaineer, St. Louis, Xenia,
Brant, Kannarrah, Central, and Wateree). The nearest trap rock is half a
mile or more distant, a columnar dike perhaps fifteen feet in thickness,
cutting the limestone vertically. On either side of this dike is a vein
from one to three feet in thickness, of white quartz with specks of ore.
Where did that quartz come from? From the limestone? But the limestone
contains very little silica, and is apparently of normal composition
quite up to the vein. From the trap? This is compact, sonorous basalt,
apparently unchanged; and that could not have supplied the silica
without complete decomposition.

I should rather say from silica bearing hot waters that flowed up along
the sides of the trap, depositing there, as in the numerous and varied
veins of the vicinity, mineral matters brought from a zone of solution
far below.

To summarize the conclusions reached in this discussion. I may repeat
that the results of all recent as well as earlier observations has been
to convince me that Richthofen's theory of the filling of the Comstock
lode is the true one, and that the example and demonstration of the
formation of mineral veins furnished by the Steamboat Springs is not
only satisfactory, but typical.

* * * * *

[NATURE.]

H