ransome.] PERSISTENCE OF THE FISSURES. 61 
in the Silverton quadrangle, no very close estimate can be made of 
the average thickness of the volcanic accumulations over the quad- 
rangle as a whole. It may provisionally be estimated, however, as 
from 5,000 to 6,000 feet. As no traces of any extensive later deposits 
have been found in the San Juan, it may be assumed that this thick- 
ness represents practically the total deposition from the beginning of 
Tertiary time. As most of the lodes worked at the present day occur 
within the volcanic series, and often from 1,000 to 3,000 feet above its 
base, it is evident that the portions of the fractures now accessible 
must have been formed at geologically moderate depths — that is, prob- 
ably under (J, 000 feet. When, further, it is remembered that erosion 
proceeded concurrently with the Assuring and probably made rapid 
headway during the progress of ore deposition, it appears that many 
of the ore deposits must have been formed well within depths fre- 
quently reached by mining operations. It might be expected that 
under these circumstances there would be a recognizable, although 
probably not very intimate, relationship between the ore as originally 
deposited and t lie present topographic surface. The full discussion 
of this subject, however, brings up questions whose solution properly 
belongs in another place, and which are treated in the section on the 
ferigin of the lode and stoek ores. 
PERSISTENCE OF THE FISSURES HORIZONTALLY AND IN DEPTH. 
Fissures are not of indefinite extent either horizontally or vertically. 
'No fixed limit, however, can be assigned to the length which a given 
fissure may attain. It depends upon the magnitudeof the stress that 
produced the Assuring and upon the relative movement which has 
taken place between the walls. Great relative movement results in a 
long fissure, but as profound faulting appears to be usually not favor- 
able to the subsequent formation of a lode, there is a certain variable 
limit beyond which length is to be regarded as an unfavorable factor 
in the productiveness of a fissure lode. There are, however, certain 
notable exceptions to the foregoing general rule that should not be 
ignored. Thus the Merrifield-Ural lode, near Nevada City, Cal., occu- 
pies, according to Lindgren, a thrust-fault fissure of probably over 
1,000 feet throw. 1 In the Silverton quadrangle fissures vary greatly 
in length. Those having a length of 2 or 3 miles are certainly not 
uncommon, ami it is very probable that some of the fractures extend 
continuously for as much as 6 miles. That great length is not neces- 
sary for the formation of a productive deposit is shown by the occur- 
rence of such lodes as the Iowa, Steezner, and East Iowa, which can 
scarcely be longer than a quarter of a mile, and which die out at their 
southern ends in small branching fractures. The exact length of a 
fissure is, of course, rarely determinable, as practical exploitation sel- 
' dom follows a lode to its total disappearance. 
— __ _ — 
1 Gold-quartz veins of Nevada City and Grass Valley districts; Seventeenth Ann. Rept. U. S. 
3eol. Survey, Pt. II, 1896, p. 167. 
