54 G. F. Becker—Teuture of Massive Rocks. 
inches possesses as good a porphyritic structure as a true erup- 
tive rock, just as granular patches may sometimes be found in 
typical porphyries. 
It would appear then that a porphyritie structure should be 
the rule among rocks which have attained a high degree of 
fluidity, while a granular structure should be characteristic of 
metamorphic rocks. So far as geologists are fully agreed, this 
is true, for opinions are divided as to the origin of granite and 
allied rocks. 
~ Many geologists, and lam among the number, do not ques- 
tion that there are both eruptive and metamorphic granites, 
diorites and diabases. If this is granted, or assumed to be true 
for the sake of argument, either the intrusive, granular rocks of 
typical habitus have been as fluid as lavas, and therefore rep- 
resent the extreme case in which various reactions liberate heat 
at exactly the same rate; or else, ike the metamorphic rocks, 
sometimes quite indistinguishable from them, they have never 
been sufficiently fluid to reach substantial homogeneity. Of 
these two suppositions the former seems very improbable, be- 
cause granular rocks are not exceptionally rare and must have 
been formed under widely prevailing conditions. ‘The alterna- 
tive supposition, that granular rocks as a whole have never 
been thoroughly fluid, does not represent a strange chemical 
equilibrium and is not @ priorz improbable. 
The rapid variation in texture and mineral composition of 
many granites is familiar to all geologists and they can draw 
their own conclusions from the evidence which this rock pre- 
sents as to the former fluidity of the granitic magma. Scheerer 
and others long ago maintained that granite had never been 
raised to a high temperature or been reduced to a cor dition of 
perfect fluidity. I prefer to employ as an illustration the dio- 
rite of Mt. Davidson, which forms the foot wall of the Com- 
stock lode in the mines known as the central group, between 
Spanish ravine and Bullion ravine. It forms a bare, unbroken 
exposure from the top of the mountain to the croppings of the 
lode, a vertical distance of 1300 feet and has been open to my 
observation at several points in the mines down to a depth ot 
3,000 feet below the croppings. The horizontal distance be- 
tween the two ravines is about three-fourths of a mile. Geolo- 
gists have perhaps seldom had an opportunity of examining a 
single rock-mass of this kind over a vertical distance of 4,800 
feet within a horizontal distance of just about one mile. The 
exposures in the mines are no longer accessible and will prob- 
ably never again be opened up. 
From top to bottom this mass presents the same general 
character. The earlier visitors to the region called this rock 
granite, but as it contains little or no quartz, Baron von Richt- 
