440 REVIEWS 
from the interior of the earth, and (4) by chemical activity between the 
water and the material in the rocks. The waters may come under 
pressure (1) by increasing gravitative stress due to increased head as 
they gain in depth, (2) by expansion in confined space as a result of 
heat, and (3) by gaseous pressure, itself perhaps the result of chemical 
activity. With all these sources of heat and pressure available, the 
important point is to discover which ones have probably been opera- 
tive to the largest extent. To the reviewer it seems clear that, as 
pointed out by Vogt and Kemp, the intrusion into the outer crust of 
the earth of molten rock from below has probably been the largest 
single factor in heating the underground waters. The distribution of 
the mineral districts in connection with the distribution of these erup- 
tives and of hot springs affords a powerful argument for this conclu- 
sion. The close association of ore deposits, eruptive rocks, and hot 
springs has been so often pointed out as to need no further citation. 
The exceptions, such as the lead and zinc deposits of the Mississippi 
valley, have uniformly been found on careful study to differ in genesis 
from the more common type of sulphide ore deposits exemplified in 
the usual occurrence of the precious metals in the Rocky Mountain 
region. 
A body of molten rock, forced up into the cooler mass above, must 
produce a profound disturbance of the ordinary underground circula- 
tion. Its quickening and vivifying influence is bound to be stupen- 
dous, and the marks of the intrusion are apt to be permanent. ‘The 
underground waters of such a district will be warm for centuries after 
the magma itself has consolidated and become fixed in form and place. 
An instructive instance, and one believed not to be exceptional, is that 
of Idaho Springs, in Colorado. There are here hot springs which pre- 
sumably are the last surviving mark of volcanic activity in the district. 
The youngest rocks present are certain andesitic dikes, whose age is 
approximately fixed by andesitic débris found in Tertiary beds near 
the mouth of Clear Creek Canon, at Golden. Since the intrusion of 
these dikes the whole country has been profoundly sculptured, and the 
interval must certainly have been a very long one, yet the underground 
waters, as shown by the hot springs, still penetrate to some portion of 
the uncooled rock. ‘The ore bodies of the district are closely related to 
these andesites ; indeed, the veins usually occur along the contact of 
the andesite and the Archean country rock, and in places are merely 
mineralized portions of the andesite itself. So far as is known there 
