

ORE-FORMATIONS 239 

gradually diminishing veins; or they may break up into a 
perfect network of strings and veinlets. 
The actual depth to which the most persistent lodes may descend is 
not known. From several considerations, however, it may be inferred 
that the fissures occupied by lodes must in many cases have traversed a 
very great thickness of rock. Since those fissures were formed there has 
been excessive denudation, whereby a thickness of rock, to be measured 
in some cases by thousands of feet or even of yards, has been removed. 
Hence the present surface where such lodes crop out is very far below 
the surface that existed when the fissures were being filled with their 
ore-formations. In the case of certain lodes which have been mined to 
a depth of 1000 yards, it has been estimated that the original surface may 
have been 2000 or even 4000 yards higher than the present, which would 
give an original vertical range of 3000 or 5000 yards at least for the lodes. 
It is doubtful, according to some, whether any cavities could be 
formed, or, having been formed, could remain open under the enormous 
pressure of 15,000 feet of rock. Others, again, have questioned the 
possibility of chemical precipitations from aqueous solutions taking place 
at such depths, where the pressure must be excessive and the temperature 

FIG. 87.—SIMPLE LODE SHOWING MASSIVE STRUCTURE. 
considerably above that of boiling water. But, as Vogt has pointed out, 
mineral deposits have certainly been made from solutions at a much 
higher temperature than is likely to obtain at a depth of 15,000 feet 
below the surface. He instances the occurrence of cassiterite, topaz, 
tourmaline, apatite, and other minerals in the pegmatite-veins of granite, 
which, having been abstracted from the granite magma, must have been 
deposited from solutions at a higher temperature than the critical 
temperature of water—6g90° F. or thereabout. 
As vertical fissures of all kinds tend to die out upwards, while the 
amount of displacement caused by faults diminishes in the same direction, 
