MaAkc#H 15, 1901.] 
tion is difficult to recognize. It is sometimes 
called ‘gouge’ by the miners. The main frac- 
ture plane follows this dike, generally within 
its walls. The principal ore-chimney is a zone 
of crushed quartzite around the intersection of 
this dike and fracture plane with the first 
granite porphyry dike, where it approaches most 
closely to the second. All the dikes show 
evidences of deformation since their intrusion. 
Smaller ore bodies are found in the hanging 
wall of the dike to the northward and gener- 
ally only a few feet in width, while the greater 
ore chimney, which is divided into four 
parts by the intersecting and practically barren 
dikes, has an area of about 200 feet square in 
horizontal section. The ore is an intensely 
crushed quartzite, recemented and largely re- 
placed by silica, so that often nothing of the 
original granular structure of the quartzite is 
distinguishable. The unoxidized ore shows, 
besides quartz, only a little finely divided py- 
rite, very little microscopic chalcopyrite and 
some telluride in spots.. The interesting fea- 
ture of the mine is the variation in the tenor of 
its ore, whose values are mainly in gold, the 
bullion being only 300 fine. In a broad general 
way it may be said that the values increased 
downwards to the 700-foot level from $20 to 
$40 and often to $70, to the ton, small lots 
running very much higher. From there down- 
wards the values have decreased to about $6 
or $8 at the tenth level, and to only $1 to $3 
dollars at the fifteenth and sixteenth levels. 
It is evidently a case of enrichment by gradual 
leaching down of the precious metals, but as 
the country is so dry that there is never any 
moisture in the mine, it must be assumed that 
this leaching took place in an earlier geologic 
period, when there was greater precipitation ; 
presumably during the Bonneville period. 
The Horn silver mine, in southwest Utah, 
occurs in a region of very much more compli- 
cated geological structure. The Grampian 
Hills at the south end of the San Francisco 
Mountains consist of more or less erystalline 
Paleozoic limestone into which a monzonite 
mass (locally called syenite) has been intru- 
ded in stock-like form. A broad contact 
zone between limestone and monzonite (locally 
called andesite) is made up of a dark brown 
SCIENCE. 
427 
rock, consisting mainly of garnet with many 
other contact minerals, notably, a white, fibrous 
tremolite (locally called needle spar). 
Opposite the mining hamlet of Frisco, an 
east and west fault has cut through limestone 
and monzonite, raising the latter so that it 
abuts against the former. Along the east base 
of the hills is a later fault plane, running mag- 
netic north and south, along which more re- 
cent andesitic breccias form the hanging-wall, 
and limestone or monzonite, as the case may 
be, the foot-wall, the fault plane having a steep 
dip to the eastward. It is this fault plane that 
constitutes the Hornsilver vein. The fault fis- 
sure or zone varies in width from a maximum 
of 90 feet down to one or two feet, and has 
been opened to a depth of 1,600 feet. It is by 
no means all ore, but consists in great measure 
of crushed wall-rock, limestone or andesite, as 
the case may be, but so much altered that its 
original character is difficult to determine. 
The ore bodies which have been of great size 
were largely replacements of this material. It 
is well known that in its early history, about 
1880 to 1885, the mine produced enormous 
masses of rich silver-lead ores and paid some 
four millions in dividends in spite of high costs 
of production, due to its situation in’ the midst 
of the desert. 
The interesting feature to which the speaker 
called attention is that whereas neither copper 
nor zine was recognized as a constituent of the 
ore in the upper levels, the main values of late 
years have ,been found in a very rich body of 
copper ore, largely copper glance, extending 
from 650 down to 750 feet. Moreover, at 500 
feet, zinc minerals began to show in small 
amount, and now in the lower levels the largest 
ore masses carry 40 to 50 per cent. of zinc, 
with 6 or 8 per cent. of silver, it being esti- 
mated that they have 300,000 tons of this ore 
in sight. In the deeper part of the mine, while 
the fault zone holds its width in the main chim- 
ney, the ore values have shrunk below the 
workable point. This is evidently another in- 
stance of the leaching down and concentration 
into the middle levels of the mine of the more 
soluble salts of copper and zinc, and their 
reprecipitation in more or less segregated 
bodies. 
