APPENDIX—GEOLOGICAL SURVEY. 
467 
of a vein. Beneath this cap rock we find an aggregation of 
mineral matter, such as galena, blende, calamine, iron pyrites, 
calc, and sometimes heavy spar chemically deposited, arranged 
as in a true fissure vein. If these strata were tilted up to an 
acute angle, but few would be able to distinguish between this 
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form of deposit and the forms of deposit in true fissure veins. 
These flat openings are important forms of ore deposits; 
they extend sometimes to two or three hundred feet in width, 
and from one-half to one mile in length, along the course of 
the fissures; indeed, they seldom become fully exhausted of 
all their minerals; the lead may be replaced by zinc, or iron 
pyrites, or spar, so as not to pay expenses, but as a vein it. 
continues, though poor. 
A good example of this form of deposit is found at the Lin¬ 
den mines, in Iowa county. Here the ore deposit commences 
in the lower portion of the galena limestone, and following the 
fissures down through “ flats and pitches ” (the peculiar form 
these fissures take in this portion of the strata) into the blue 
limestone, it spreads out into broad horizontal sheets of ore for 
several hundred feet wide, and has been worked continuously 
for'nearly a mile in length. The ore in this mine is not con¬ 
fined to “flat openings,” (the usual form in the blue lime¬ 
stone,) but is often found filling the fissures as they extend 
from one bed of rock to another. In this way the deposit of 
ore that commenced in the galena limestone has worked its 
wav down into, and almost through, the blue limestone; in 
fact, entirely through, if we regard the few feet resting on the 
sandstone to be magnesian limestone. In one or two places it 
has been followed down to the sandstone. 
This mine has yielded not less than twelve million pounds 
of lead ore, and several million pounds of zinc ore, and if 
owned by an enterprising company and drained by an adit to 
its present depth, would no doubt yield many millions more. 
I refer to this only as an example of this class of fissures and 
form ol ore-deposits, and to show that the same fissures vary 
in form in passing through the different beds of rock in the 
same formation, consequently the forms of ore-deposits vary 
