44 ECONOMIC GEOLOGY OF SILVERTON QUADRANGLE, [bull.182. 
consistently used throughout this report, as a more general term than 
vein, to designate either a simple filled fissure or a zone of closely 
spaced Assuring with possibly more or less impregnated or replaced 
country rock. Lode thus includes what Yon Cotta and Yon Grod- 
deck 2 have called complex veins (zusammengesetzte Gauge). 1 
A similar usage has been followed b} 7 Emmons and Tower 2 in their 
description of the ore deposits of Butte, Mont., and to some extent 
by Purington 3 in describing the ore deposits of the Telluride quad- 
rangle. 
DISTRIBUTION OF THE ORE-BEARING FISSURES. 
Fractures in the rocks, containing more or less ore, are widely dis- 
tributed within the quadrangle, but are larger, more abundant, or 
richer in metalliferous contents in certain limited districts than else- 
where. Prospects occur thickly dotted over the whole area, but the 
important mines are found in more or less isolated groups. A partial 
idea of the distribution of the fissures may be gained from the accom- 
panying map (PI. Ill), on which but a small proportion of the actual 
fractures are indicated. Moreover, in plotting the lodes the smaller, 
poorly exposed, and apparently nonworkable ones must necessarily 
be often disregarded, and the map can not therefore be taken as show- 
ing strictly the true distribution of all the fissures, but only of those 
which are superficially conspicuous or which carry ore in sufficient 
quantity to be worked or prospected. As the map suggests, fissures 
carrying variable amounts of ore are less noticeable features in the 
southwest and southeast corners of the quadrangle than elsewhere. 
But they occur in all the rocks which possess any considerable distri- 
bution within the area, from the Algonkian (or Archean?) schists, 
which constitute the basement formation of the region, to the lalesi 
monzonitic intrusions that cut the Tertiary volcanic series. By far 
the greater number of them, however, are found in the volcanic rocks 
of the Silverton and San Juan series. This is apparently due chiefly 
to the fact that these rocks occupy the greater part of the quadrangle 
rather than to any special determining factor in the rocks themselves. 
It is admittedly difficult, however, to isolate such a possible factor 
from others more directly associated with the origin and accumulation 
of these rocks than with their texture and structure. Whether the 
productive ore bodies that fill some of the fissures are as impartially 
distributed in the various formations as the fissures themselves is a 
different question and will be discussed in another place. 
The existence of local areas of especially pronounced Assuring and 
mineralization has alread} 7 been pointed out. Silver Lake Basin may 
be cited as a center of one such areal group of fissures. Not only are 
1 Die Lehre von den Lagerstatten der Erze, Leipzig, 1879, p. 35. 
2 Geologic Atlas U. S., folio 35, Butte special. 
3 Preliminary report on the raining industries of the Telluride quadrangle, Colorado: Eight- 
eenth Ann. Rept. U. S. Geol. Survey. Pt. Ill, 1898, p. 772. 
