PROCEEDINGS 
OF THE 
NATIONAL ACADEMY OF SCIENCES 
Volume 2 MARCH 15. 1916 Number 3 
THE MECHANICS OF INTRUSION OF THE BLACK HILLS 
(S. D.) PRE-CAMBRIAN GRANITE 
By Sidney Paige 
U. S. GEOLOGICAL SURVEY. WASHINGTON. D. C. 
Received by the Academy. January 25. 1916 
The Pre-Cambrian rocks of the Black Hills consist of a great series of 
slates and schists, for the most part monotonously alike, striking in a 
northwest direction and having steep dips generally, except in the extreme 
southwest, to the east. Close study shows that the persistent east- 
ward dips represent both schistosity and bedding, the two for the most 
part parallel; and that the series is compressed into a number of great 
folds which comprise innumerable minor isoclinal folds. A sufficient 
number of individual beds have been traced to locate the position and 
nature of the greater axes of folding, and to locate two important 
faults. 
To the south great intrusions of granite break through the strata, 
and around the principal mass, forming Harney Peak, a notable schis- 
tosity is developed parallel with the granite contact, and superimposed 
upon the normal schistosity of the region. 
Field relations indicate that this granite came into its present position 
in the main by physical distension of the invaded rock body, under 
great load, and that it thus modified to an important degree the normal 
process of regional compressive folding, forcing the schists into closely 
appressed recumbent folds, parallel with the advancing surface of the 
magma. The rocks were deformed; they yielded to the advance of 
the magma, and the schistosity produced by this movement and fold- 
ing lent itself to the further injection of the granite by numberless 
parallel dikes and by lit-par-lit intrusion. The harder rocks, the quart- 
zites, were distended and broken apart by the upward movement, and 
the magma flowed in between the blocks. 
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