ransome] INTERSECTIONS AND RELATIVE AGES OF FISSURES. 59 
filling-, is taken to indicate relative movement of the walls, not neces- 
sarily resulting in any great net displacement. In other words, they 
are fault planes of greater or less movement, possibly oscillatory. The 
width of these post-mineral fractures, as they are sometimes con- 
veniently called, may vary from a narrow, scarcely noticeable clay 
seam, up to zones 12 feet or more in width, in which the gouge is 
mingled with masses of shattered country rock, and of which the most 
notable example is found in the zone of so-called "broken ground" 
in the Silver Lake mine (seep. 154). Occasionally the fine clayey mate- 
rial is lacking and the fracture is the course of a copious stream of 
water. Such a fissure was encountered in the Silver Lake mine in 
crosscutting to the New York City vein on level C, and the same or a 
similar water-bearing fracture was subsequently cut in the Unity tun- 
nel about 400 feet lower. Since these post-mineral fractures can be 
studied, as a rule, only in the vicinity of the lodes, and as they are 
frequently directly connected with the latter, they may be conven- 
iently classed as follows: 
(1) Strike fractures generally parallel with the lode. — These are most 
commonly seen as gouges next the hanging or foot wall or in the 
country rock near to and approximately parallel with the lode in strike 
and dip. Gouge seams of this class, although not generally charac- 
teristic of the lodes of the Silverton quadrangle, are frequently well 
exhibited. They are found on both foot and hanging wall of the Silver 
Lake lode, particularly the latter, and also in the country rock for a 
distance of several feet on either side of the main fissure. When such 
later fractures have followed a previously existing vein or stringer 
the}^ usually contain crushed vein quartz mingled with the gouge. 
(2) Strike fractures dipping at considerable angles with the lode. — 
These are not so common as the foregoing, but sometimes occur. The 
ore in this case will be found to rest locally on a bench or step of 
country rock, separated from the latter by a claj^ seam of varying 
thickness. An example of this structure was noted in a stope on the 
Silver Lake lode (see p. 152, fig. 11). Such an occurrence needs to be 
carefully distinguished from the steps or benches of ore formed by the 
original opening of a somewhat irregular fissure by thrust faulting. 
In this case there should normally be no gouge between the ore and 
the country rock upon which it rests. 
(3) Cross fractures differing from the lode in both strike and dip, 
and cutting it at an appreciable angle. — These are observed in nearly 
all extensive workings, and are sometimes fairly abundant and regu- 
lar, as in the case of those which cut the Silver Lake lode (p. 151). 
Besides the three classes noted, various irregular fractures occur in 
almost all lodes which can not be definitely grouped. 
The post-mineral fractures are simply of local importance to indi- 
vidual deposits, and it has not p roved practicable to discuss and 
classify them with reference to the quadrangle as a whole. Their 
