GRANITE ROCKS OF BUTTE, MONT., AND VICINITY 745 
largest mass is 114% miles by 2,, miles, and is known from mine 
workings to be several hundred feet thick, resting on the Butte 
granite. Besides this large mass there is another of about one- 
third the size and numerous smaller bodies, as outlined by the 
author on the geologic map of the Butte district. In the cases 
hitherto observed by the writer, and those commonly described, 
aplites occur in dikes commonly quite narrow, but often of con- 
siderable length; such masses have been supposed to be the fill- 
ing of cracks formed in the cooling granite, the aplite magma 
coming either from an acid residuum or nucleus of the mass or, 
as suggested by Turner, a product squeezed out of the crys- 
tallizing granite and gathered in cracks due to its shrinkage. A 
study of the Butte aplites shows that, though the dikes of this 
material may owe their origin to some such cause, that the irregu- 
lar lense-like or meniscoid masses are sometimes local bodies 
unconnected with any feeder. The inference derived from a 
careful examination of many exposures is that the material is 
due to some such process as that suggested by Turner—a sort 
of segregation. In the description of the remarkable differen- 
tiation zone of Square Butte’ it was shown that the basic outer 
part of the intrusion, itself a product of differentiation, holds a 
thin band of white syenite due to further separation or differen- 
tiation of the feldspathic constituents in the crystallizing mass. 
This hypothesis, my belief in which has been strengthened by 
further observations of other laccoliths in the same region, seems 
to explain the manner of occurrence of the aplites in the Butte 
mass. It is believed, upon evidence which cannot be presented 
here, that the Butte district is on the downthrown side of a fault 
and that its granitic rocks represent the upper part of the batho- 
litic intrusion. In this uppermost part of the intrusion partial 
differentiation is believed to have taken place, the normal granite, 
represented by analysis No. 516, splitting up into the more basic 
phase represented by the Butte type and the acidic type, the Blue- 
bird aplite. This hypothesis demands that as the Butte granite 
* WEED and Prrsson: Highwood Mountains of Montana, Bull. Geol. Soc. Am., p. 
406, 1896. 
