raxsome.] THE LODE FISSURES. 63 
ORIGIN OF THE FISSURES. 
The attempt to explain the origin of so complex a network of fis- 
sures as occurs in th< Silverton quadrangle presents many difficulties. 
No explanation that can be at present proposed is to be regarded as 
complete or as free from various necessary assumptions. Our knowl- 
edge as to the degree of homogeneity of the various rock masses under 
strain, the depth at which the fissures were originally formed, their 
exact extent, direction, and distribution, the geological conditions 
obtaining at the time of Assuring, and numerous other essential data 
are far too fragmentary to permit of anything like rigid analysis lead- 
ing t<> irrefutable results. And yet the problem is not so hopelessly 
insoluble but that it may be reasonably taken up in the hope of find- 
ing at least a working hypothesis to account for major features. 
Fissures in which ore deposits occur are very commonly regarded as 
produced by tangential compressive stress transmitted in nearly hori- 
zontal planes by the rocky envelope of the earth, within what Van 
llise 1 has termed the zone of fracture, limited by a depth of 10,000 
meters. In many dist ricts t his is, without much doubt, the immediate 
cause of the fracturing. Thus, in the Grass Valley and Nevada City 
region of California the ( jugated systems of flatly dipping regular 
lodes, opened with the accompaniment of more or less thrust faulting, 
were probably, as Lindgren concludes, "produced by a succession of 
compressive stresses applied in different directions, chiefly from east 
to west and from north to south." 2 
In accounting for the fissures of the Telluride quadrangle, Puring- 
ton 3 arrived at somewhat similar conclusions. lie states: 
From all observed phenomena it seems probable that the Assuring was made by 
forces acting at time intervals not far apart and at localities not far removed from 
one another. Since the Assuring is later than all the rocks of the quadrangle 
[Telluride], volcanic disturbances whose product is now visible can not be cited 
to account for it, but it is entirely possible that later disturbances of volcanic 
nature, which did not result in surface flows of lava, have produced a straining to 
the point of rupture in the tract under consideration. It is thought, with great 
reason, that in the area directly east and northeast of the Telluride quadrangle 
there are centers and necks of volcanic eruption. Such evidence as has been col- 
lected in the present investigation points to those quarters as the source from 
which the pressure came. 
Since the fissures of the Telluride and Silverton quadrangles were 
formed at the same time and undoubtedly had a common origin, 
Purington's results have an important bearing on the problem in hand, 
and it becomes necessary to scrutinize them closel} 7 . The geological 
mapping of the Silverton quadrangle and reconnaissances to the north 
of it, accomplished by Cross and Spencer since Purington's report was 
written, have failed to identify the actual "centers and necks of 
1 Loc. cit., p. 589. 
2 Loc. cit., p. 170. 
3 Eighteenth Ann. Rept. U. S. Geol. Survey, Pt, III, 1898, p. 770. 
