BAnsome.] ORIGIN OF THE FISSURES. • 65 
fcical, but horizontal, then tangential stress might produce systems of 
nearly vertical fissures intersecting nearly at right angles conjugate 
fissures. As shown by Daubree's l classic experiments, such fractures 
would be undulating, no1 strictly parallel, and might branch as do the 
fissures of the Silver Lake Basin. Moreover, as shown by Daubree's 
illustrations, the walls of such fissures will not necessarily be always 
slickensided, nor need the faulting be in all cases perceptible. Such 
faulting as look place would result mainly in horizontal displacement, 
>r offset, and, not being necessarily great in amount, might be easily 
>\erlooked. It is possible in this way to account for much of the 
assuring of the Silverton quadrangle, and perhaps this view best 
explains the two dominant directions of Assuring intersecting nearly 
it righl angles. 
This hypothesis that the dominant northeast-southwest and north- 
west-southeast fissures were formed by compressive stress, acting 
learly horizontally, all hough the direction of least resistance which 
t presupposes is not thai which would appear most probable, seems 
jo offer fewer serious objections than any other thai has been devised. 
Fissures of contraction fchey certainly can not be, for they cut indif- 
'erently rocks of most diverse age, character, and origin. Fissures 
originating from tensional si rains would probably be more irregular 
in<l would nol be associated with close parallel sheeting of the rock. 
Tension would find relief in a single fissure rather than in a series of 
slosely spaced parallel fractures. Shearing strains due to differen- 
ial elevation or subsidence might culminate in fissures, but these 
jould not be expected to show the general regularity and the per- 
sistency of the principal fissures of the Silverton quadrangle, and 
vould probably reveal evident faulting. Faults probably due to such 
forces occur within and about the quadrangle, but they rarely con- 
ain ore deposits. Torsional stress, which by Daubree 2 is regarded 
is a system of pressures, and by Becker 3 as a system of tensions, is 
•apable of producing systems of parallel iissures intersecting at 
learly 90° as well -as radial fissures. It may at times be impossible 
o distinguish such fissures from those formed by simple pressure. 
Jul the fissures produced by torsion are in general decidedly curved, 
villi a more strongly marked radial disposition. Their character- 
sties are given in detail by Becker in the paper cited. In a homo- 
;eneous block subjected to pressure, there are formed, in addition to 
he main conjugate fractures, various more or less irregular fractures, 
aaking angles of less than 45° with the direction of pressure, and 
requently branching from the main fissures (see Daubree, loc. cit., 
> 1. II). Such fractures are comparable with the branching fissures 
f Silver Lake Basin and Placer Gulch. For this reason, and on 
1 Etudes Synthetiques de Geologie Experimental, Paris, 1879, PL II. 
2 Loc. cit., p. 321. 
3 Torsional theory of joints: Trans. Am. Inst. Min. Eng., Vol. XXIV, 1894, p. 137. 
Bull. 182—01 5 
