APPENDIX—GEOLOGICAL SURVEY. 
463 
mineral formations as their cause, and especially mineral dis¬ 
tricts of large extent, can never be explained by local observa¬ 
tions of limited extent. 
On the character of the rocks in this district I need say but 
little ; this, with their geological relations, is full}" described in 
the report of 1862. I will state, however, for the benefit of 
those who have not seen that report, that the strata of 
the lead district, so far as exposed by mining, consists of a bed 
of limestone, known locally as the galena limestone, but chem¬ 
ically as a dolomite or magnesian limestone about 250 or 300 
feet thick; and a bed of compact fossiliferous limestone, known 
locally as the blue limestone, but in its geological order as the 
Trenton limestone. These strata are of the lower silurian age, 
but are comparatively undisturbed by either plutonic or meta- 
morphic action; that is, there are no elevations of granite or 
trap, or any other igneous rocks protruding through these 
strata. 
The fissures traversing these strata are not like those in 
which we find what are called true fissure veins, such as are 
met with in crystalline rocks of plutonic or rnetamorphic ori¬ 
gin, but fissures peculiar to this formation in similar lead and 
zinc districts in different parts of the world, and which belong 
to that class of fissures I have denominated hydroplutonic. 
The fissures traversing the galena limestone are usually ver¬ 
tical, or nearly so. The ore is sometimes found filling the fis¬ 
sure, where it is small, with little or no other matrix than the 
limestone walls against which it is formed. In this condition 
it forms a sheet of ore (as the miners call it) from 1 to 20 
inches thick, enclosed firmly in the rock. When the fissure is 
wider, and its sides show evidence of decomposition, the ore is 
usually found in a clayey matrix lined with ochre. 
The larger deposits, however, are found where the rocks be¬ 
tween two or more fissures have been decomposed, and are 
called by the miners “ crevice openings.” This decomposition 
usually takes place beneath a harder portion of the rock, as 
though greater resistance had been offered here to mechanical 
forces acting from below, and a gentle lifting of the strata had 
