52 ECONOMIC GEOLOGY OF SILVERTON QUADRANGLE. [bull.182. 
ore in lodes. It remains to consider another manner in which the fis- 
sures of this region are sometimes localty grouped, not in parallel, 
but in more or less radial, or, perhaps more accurately, branching, 
arrangement. This feature is well shown by the lodes of Silver Lake 
Basin. The accompanying diagram (fig. 2) is a horizontal sketch pro- 
jection of the productive fissures worked in the Silver Lake and Iowa 
mines, compiled from maps of the underground workings. The rela- 
tions of the lodes here shown are described on pp. 148-149, and it is 
only necessary in this place to call attention to the branching arrange- 
ment of the fissures and to the fact that all these lodes appear to have 
been formed at the same time. Another case of conspicuous branch- 
ing occurs at the head of Placer Gulch, where several lodes having 
an average strike of about N. 25° E. split off from the main Sunnyside 
lode. The junctions in this case have not been exposed underground, 
but from the similar character of their filling it is highly probable 
that these fissures, like those of Silver Lake Basin, were all filled 
at the same time, and were therefore probably formed practically 
simultaneously. 
Still a third and interesting case of branching fissures is that at 
Lake Oomo, partially and somewhat diagrammatical^ represented or 
the map (PI. III). A view T of some of these lodes is shown in PI. VIII 
In this instance, again, the lodes are all of similar character and al 
appear to have been formed contemporaneously. 
Another mode of coordination sometimes observed in groups of fis 
sures is that which Lindgren, 1 following Daubree, has describe* 
under the name of conjugated fissures — i. e., in the particular regioi 
described by Lindgren, fissures having generally parallel strikes bu 
dipping symmetrically in opposite directions. In the Silverton quad 
rangle, however, the fissures usually stand so nearly vertical tha 
such a relation, if it exists, can not be clearly made out. The diffei 
ence of dip, which would determine to which subdivision of sue 
a conjugated system any given fracture would belong, is so sligh 
and may be so frequently found in various portions of one and tb 
same fracture, that it does not seem practicable to recognize sue 
systems. The low angle of dip at which most of the Grass Valle 
and Nevada City lodes lie makes the conjugated character of tl 
fissure systems readily recognizable. It is to be noted, howeve 
that in such conjugated fissures as those above described it 
assumed that the rocks, in fracturing under tangential stress, yield* 
in a vertical direction, and that the prisms consequent upon tl 
rupturing lie horizontal. But it is obvious that if, for some caus 
the rocks could yield more easily in a horizontal direction, at rig 
angles to the direction of thrust, then the resulting prisms wou 
stand vertical and the conjugate fissures would, in this case, form 1a 
1 The gold-quartz veins of Nevada City and Grass Valley, Cal. : Seventeenth Ann. Rept. U 
Geol. Survey, Pt. II, 1896, p. 164. 
