174 The American Geologist. September, 1902 
schistosity. The underground workings show the veins to 
vary somewhat in width and in the relative abundance of in- 
cluded rock-fragments, when they pass from one belt of feld- 
spathic gneiss to another, and more markedly when they pass 
into the more schistose rocks; but the change is so abrupt 
where the amphibolytes are encountered that the miners say 
the vein is faulted. In fact, the tough nature of this rock has 
sometimes deflected the vein, and has always, in the instances 
observed, narrowed it from 7 to 8 ft. in width of good ore to a 
foot or more of barren gangue. Fig. 2 shows an example.* 
(The same phenomenon is described by De la Beche.t The 
vein shown in Fig. 3 encounters an elvan dike b, in passing 
from d to e, and passes up the wall of the dike, scarcely show- 
ing as a vein alongside of the dike until it crosses the dike at c, 
and continues on through the slates. ) 
At Neihart, where the vein passes from schi.st into Pinto 
dioryte, a peculiar igneous rock of coarse grain and texture, it 
Fig. 3 
b 
Fissure Deflected by Dike (De la Beche.) 
invariably narrows, but is well-defined. In the rhyolyte por- 
phyry the same fissures lose their compact character, ramifying 
into a network with shattered rock between, which dissipates 
the ore ; so that, while the surface-workings are commonly 
rich, the ore-body does not pay in depth, owing to the large 
amount of waste. 
At Butte. Montana, the veins occur in coarse-grained gran- 
itic rock (a quartz-monzonyte ) with intrusions of aplyte gran- 
ite and dikes of porphyry. Where the veins cross the porphyry 
they are narrower to a marked degree ; and the same is true 
of the aplyte. In part this is due to a more intense metaso- 
matic replacement of the granite than of the other rocks, — a 
fact which will be discussed in describing the influence of the 
wall-rocks on the filling. It is quite evident that the rock- 
character has influenced the fissure. 
• From the 20tb Ann. Report of the V. S Geol. Survey, part iii, p. 421. 
t Geological Observer, p 657. 
