62 ECONOMIC GEOLOGY OF SILVERTON QUADRANGLE, [eull.182. 
The formation of fractures is limited vertically by that depth below 
which the rocks are under such pressure that no fissures can form. 
As the zone of fracture, according- to Hoskins and Van Hise, 1 has a 
depth of about 10,000 meters (33,000 feet), and as fractures may also 
form in the still deeper zone of combined flowage and fracture, it is 
plain that this limit will never be reached in mining operations. But 
many fractures undoubtedly die out long before reaching this ulti- 
mate limit. The longer horizontal and the vertical dimensions of the 
fissures were probably originally nearly the same, and it is not likely 
that the present depth of any fissure very greatly exceeds its length. 
The depth will at least be roughly proportional to length. But the 
depth to which fissures extend is rarely actually determined, as the 
value of the ore body nearly always falls below the limit of profitable 
working long before the fissure itself disappears. But that many 
smaller fissures do die out at moderate depths is a well-attested fact. 2 
In the Silverton quadrangle, where mining development is as yet 
restricted to moderate or slight depths, no Avell-authenticated case is 
yet known of the actual dying out in depth of a fissure which carried 
workable ore at a higher level. The ore may change in character, or 
may disappear, as in the North Star (King Solomon) mine, but the 
fissure still continues to an unknown depth. 
In such instances as the last many considerations enter into the 
question as to whether it is advisable to follow the pinched fissure to 
greater depth in the hope of finding new ore bodies. The first step 
in such an issue is to determine whether the pinch is merely a con- 
striction in a fissure extending to greater depth or whether it signifies 
the final diminution of the lode. This can be decided only by a care- 
ful consideration of the length and strength of the fissure as exposed 
above, of the possible faulting which accompanied its formation, and 
of the behavior of the fissure in those portions alread}^ mined. If the 
length of the croppings be several times the depth attained, if the 
fissure be usually strong, if it has been opened by faulting, and if it 
has been found subject to local pinches above, it may safely be con- 
cluded that it will persist and open out again with increased depth. 
But equally important is the question of the character of ore that may 
be found below, even if the fissure continue, for, as will be fully dis- 
cussed later on, the ore contents of a given fissure are not constant at 
all depths, either in kind or in value. Lastly, in connection with these 
factors must be considered the costs involved in mining the ore from 
an increased depth. 
The foregoing relates to simple fissures. Lodes in general exhibit 
similar characteristics, but their persistency will in the main be 
greater than that of a simple fissure. They are subject to the same 
general laws as the individual fractures of which they are composed. 
Principles of North American pre-Cambrian geology; Sixteenth Ann. Rept. U. S. Geol. 
Survey, Pt. 1, 1896, p. 593. 
2 Lindgren, loc. cit., p. 162. 
