SEQUENCE OF PROCESSES OF ALTERATION. 
195 
The changes, as far as can be ascertained by a comparison of the analyses, involve 
a great loss of soda, which is more than compensated by an increase of over 4 per 
cent of potash. This corresponds to the development of soda-free adularia instead 
of sodic microcline and to the formation of sericite in the oljgoclase. There is a 
distinct decrease of silica, which is probably carried away in soluble form. Magnesia 
is practically eliminated, while lime is slightly increased. Whether any actual 
addition has taken place is doubtful. Fluorine, vanadium, and molybdenum have 
been introduced. The iron remains constant, substantiating the statement made 
above that pyrite forms principally from the constituents of the rock. Little 
change is noted in titanium and phosphorus. 
SILICIFICATION AND COMPLETE REPLACEMENT. 
A silicification of the rocks along the veins is exceptional. It does not extend far 
from the vein, and is probably developed only in case of very porous or shattered 
rocks. 
In several mines, and often associated with rich ore, occur small quantities of a 
dark, almost black, fine-grained and hard-vein material, which consists chiefly of 
quartz and pyrite, and which is probably due to replacement of detrital material or 
mud in which siliceous solutions could freely circulate. A drusy material of this 
kind from level 3 of the Isabella mine, 100 feet north of the Lee shaft, contains 
fine-grained quartz, with a little fluorite and abundant aggregates of pyrite, zinc 
blende, and molybdenite, as well as granular dolomite. A narrow, flat vein of this 
kind in an ore shoot on level 6 of the Abe Lincoln mine, in which gneiss is the pre¬ 
vailing country rock, consists of a granular mixture of quartz and idiomorphic 
adularia, abundant pyrite, grains of carbonate associated with felted nests of 
serpentine and sericite. 
SEQUENCE OF PROCESSES. 
The changes which have taken place in the rocks may be divided into those 
which are clearly and unquestionably caused by vein formation, as indicated by 
their occurrence along the lines of the mineralized fractures, and those affecting 
larger areas and less obviously produced by the solutions circulating on these 
fissures. Both Cross and Penrose® came to the conclusion that there were two or 
more epochs of alteration by hot waters and possibly also epochs of fumarole action 
by sulphuric acid, chlorine, and fluorine contained in the magma. They agree that 
the fissures and sheeted zones in the breccia could not have been produced in the 
soft and unconsolidated rock, and that, therefore, the general cementation and alter¬ 
ation of the breccia involving the formation of pyrite and sericite took place before 
the veins were formed. 
Although this view has some supporting evidence, we can not fully subscribe to 
it. It is true that hot waters circulated through the volcanic rocks for a considerable 
period and that the solutions varied from time to time. Almost immediately after 
the explosive action had ceased the breccia must have been permeated by steam or 
superheated waters, and we hold that the rock pressure, together with the influence 
of this moisture, would have been sufficient to cement the loose mass in very short 
Sixteenth Arm. Rept. U. S. Geol. Survey, pt. 2, 1895, pp. 70, 161. 
