746 WALTER HARVEY WEED 
is but slightly more basic than the prevailing form, the pro- 
portion of aplite should be small. The field observations show 
a quantitative relation which, as far as it can be estimated, con- 
firms the view. 
This -hypothesis implies a gathering of the iron, magnesian, 
and lime molecules out of the general magma and their concen- 
tration in the quartz-monzonite, with a separation out of the 
aplitic material, richer in alumina, alkalis, and silica which did not 
form an inner kernel as it has in laccolithic differentiations, but 
local masses in the basic granite. This hypothesis has already 
been anticipated by Cross in the discussion of evidence of dif- 
ferentiation at Rosita, Colo.? 
If the Butte granite is a border or upper contact facies of 
the batholith, this separation may have been induced by contact 
cooling. Observations of many of the smaller intrusive stocks 
of the Montana mountains and of the contacts of the larger 
batholiths show that there is more or less of a mixing of basic 
and siliceous materials as if they were stirred together while 
pasty. The rocks grade into one another and there are no sharp 
contacts. 
In the large aplite intrusions there is no sahlband alteration. 
The grain continues the same in both rocks, but at a certain line 
there is a change in the relative proportions of the minerals. In 
the smaller bodies and little dikelets the grain of the aplite is 
finer, though there is no contact band or evidence of chilling. 
In the Butte area this is uncommon. There is, it is true, a sharp 
contact between granite and aplite, but there are transition forms 
and even masses of granite in the larger aplite bodies which are 
clearly not included fragments but integral parts of the mass. 
Yet there is commonly a definite separation of the two rocks, 
and it is certain that there has been no mixing of the two mate- 
rials due to convection or movement before consolidation. 
In most of the aplite bodies the grain varies considerably from 
place to place; sometimes the rock becomes a micropegmatite, 
* Geology of Silver Cliff and Rosita Hills, Colo., Seventeenth Ann. Rep. Dir. U.S. 
Geol. Surv., p. 329. 
