GEOLOGY OF THE TIN BELT. 37 
the intervention of localities where the rock is stronger; i. e., of greater cohesion between 
the two walls of the plane of movement. 
Material forced by hydrostatic pressure from a reservoir below would follow these direc- 
tions of weakness, force the walls apart, and occupy the space which it thus created for 
itself. Where cohesion had been entirely overcome by the original movement, the expansive 
force would be most effective, the intrusive material would have the greatest thickness, and 
the dike formed on solidification would have the greatest width. (See fig. G, p. 50.) 
The explanation of the irregular form of the pegmatite masses which conform to the 
schistosity of the inclosing rocks lies, therefore, in the fact that the planes along which 
intrusion or injection took place were themselves of irregular extent and their walls offered 
varying resistance to being forced apart by the invading material. 
The explanation of the form of those dikes which cross the structure of the surrounding 
rocks is somewhat different, just as their form is different. After the rocks of this region 
had been folded to their present structure and the forces which caused this folding had 
nearly or quite subsided new forces more or less transverse to the first became active. 
These new forces tended to produce folds transverse to those already existing. But the 
resistance of this folded belt to contortion across the strike was too great to be overcome, 
just as corrugated iron is rigid to deformation at right angles to the corrugations. Since 
the stress induced by these later forces could not be relieved by folding, fracture at right 
angles to the direction of force took place. This fracturing was sufficient to relieve the 
stress and little faulting or other movement followed. 
Later, when the material of the pegmatite dikes began to exert pressure on the walls of 
its reservoir, it found, besides those irregular places of weakness parallel to the foliation of 
the rocks already described, definite, sharp fractures or cracks, offering a fairly uniform 
resistance to the passage of a fluid and hence capable of being expanded and filled fairly 
evenly throughout their extent. These cross dikes are in consequence regular and persist- 
ent. In some instances relatively small branches or offshoots from the main cross dike are 
injected into the wall rock parallel to its schistosity. This adds to the evidence that 
although the cross dikes occupy fissures of later formation than those filled by the inter- 
foliated dikes, the two classes of dikes arc contemporaneous. (Compare fig. 5, p. 47, and 
fig. 6, p. 50.) 
From these considerations the conclusion may be deduced that in the exploitation of a 
dike which crosses the foliation of the inclosing rock the prospector, while proceeding by 
those cautious methods which should always attend the early development of a mining 
region, may reasonably expect to find the dike continuous and possessing a fair degree of 
regularity. In the case of the dike parallel to the structure of the country rock, however, 
he must be prepared to meet great and sudden variations in its form and position; but if 
he explores beyond the limits of one lens he may often be rewarded by the discovery of 
another. 
So far as known at present, faulting has not been sufficient to affect in any important 
degree the position or extent of these dikes. 
DESCRIPTION AND MINERALOGY. 
The tin-bearing rocks embrace two of the varieties of pegmatite described on pages 20-22. 
As was there stated, these two varieties, the third and fourth, may be simply different 
facies of the same rock. At any rate, it is highly probable that they do not differ greatly 
in composition. The general petrographic descriptions of these rocks have already been 
given. It will here be appropriate to consider them more in detail and in the light of 
ore-bearing formations. 
The third or microcline variety of pegmatite was at first thought not to contain cassiter- 
ite,« but recent developments at the Ross mine below the zone of extreme decomposition 
have shown that this rock is the one which carries the cassiterite. In several places in 
a Bull. U. S. Geol. Survey No. 260, 1905, p. 191. 
