114 ^/'<? American Geologist. August, 1902. 
of arkoses, conglomerates, qnartzytes , and shales, and some 
limestone. Portions of the arkose are metamorphosed into 
fine-grained granitic rock, with fresh biotites and feldspars. 
The arkose series is cut by dikes of more recent andesyte, bas- 
alt and tonalyte. The tonalyte consists of feldspar, hornblende 
and biotite, with occasional augite and hypersthene and mag- 
netite (?). The feldspar varies between oligoclase and by- 
townite, and the secondary minerals are actinolite, chlorite, 
epidote, pyrite, calcite, kaolin, etc. The tonalyte cuts the ar- 
kose series and the early andesytes, and is cut by the later an- 
desyte and probably also by the basalt. It is correlated with 
the Snoqualmie granite of Smith and Mendenhall.* 
The later andesytes, tuffs and breccias are the products 
of explosive eruption from volcanic vents like mount Baker, 
mount Rainier and Glacier peak, and are supposed to belong 
to a period which lasted from the late Pliocene to late Pleis- 
tocene. 
To this same period of volcanic eruption, profound frac- 
turing and slight faulting, is ascribed the origin of the joints 
which now form the loci of the ore-bodies. 
The chief ores of the Monte Cristo district are pyrite, pyr- 
rhotite, arsenopyrite, blende, galena and chalcopyrite. Rarer 
minerals are chalcocite, bornite, molybdendite and stibnite, and 
oxidation products are malachite, limonite, hematite, mela- 
conite and scorodite. 
These ores carry relatively small amounts of gold and sil- 
ver. In the JNIonte Cristo mine the average ore at some dis- 
tance below the surface carries 0.6 of an ounce of gold and 
y ounces of silver. The richer ores carry 1.4 ounces of gold 
and 18 ounces of silver. The surface ores in the same mine 
average 0.95 ounces of gold and 12 ounces of silver. The chief 
gangue minerals are quartz and calcite. Epidote and blue 
soda-amphibole are developed in the wall rock near the veins. 
There has been very little post-mineral movement in the 
ores and enclosing rocks. The ores have all been deposited 
during a period geologically not far removed from the present. 
Solution and deposition of metallic minerals in the form of 
ore deposits has been going on in very recent times, even 
since the sculpturing of the present topography. Many of the 
smaller veins and some of the larger ones, which at the surface 
* Bull Geol. Soc. Am.. XI, p. 225. 
