130 
AMERICAN GEOLOGY. 
is never a mining one, though it does not follow that all dis¬ 
turbed districts are rich in mines. But without doubt all min¬ 
ing districts have been the seat of great disturbances of the 
strata. They have been subjected to chemical as well as 
mechanical forces; strata have been crushed and displaced by 
faults, and along with those displaced strata their mineral veins 
have suffered displacement also. 
CONTINUITY AND PERSISTENCE OF VEINS IN DEPTH. 
§ 87. The expenditure of capital in mining, is warranted only 
where there is a good degree of assurance of the persistence 
of the vein in a downward direction. While it must be admit¬ 
ted that each particular case should have its own evidence of 
its continuity, still that evidence is based on general facts and 
principles. We refer in the first place to the theoretical views 
we have offered relative to the origin and formation of veins. 
If that theory is sustained by observation, we are carried a 
great way towards a satisfactory establishment of a persistence 
of veins in a downward direction; subject, however, to an inter¬ 
ruption, it may be by the operation of local causes, which may 
have deranged or interrupted the formation of the vein fissure^ 
We may not, however, neglect the teaching of facts as they 
are being furnished by the workings of the oldest mines. To 
the mining records of other countries -where it has been the 
business for centuries, we may refer with great satisfaction. The 
veins in the mining district of Cornwall, England, have been 
pursued successfully to the depth of 1800 feet. It was thought 
by one class of geologists that they had become less rich at the 
depth of 600 to 1200 feet; still they appear to retain their 
richness to day with very little or no abatement, to the depth 
of nearly 2000 feet. There are still deeper mines in the dis¬ 
tricts of the Hartz mountains. Those of Clausthal and Zeller- 
thal have been pursued to the depth of 1920 feet. Those in the 
district of Andreasberg, 2400 feet. In the neighborhood of 
Freyberg, Saxony, the mines have been worked to the depth of 
1800 feet. The Mexican silver mines, have been explored to 
