No. 5.— An Oceurrence of Harney Granite in the Northern Black 
Hills. 
By Henry G. FERGUSON AND FREMONT N. TURGEON. 
The centre of the Black Hills dome, in South Dakota, is composed 
of granite and schists, the former a pegmatitic type occurring chiefly in 
the Harney Peak region in the southern part of the hills. From this 
core the Palaeozoic sediments dip away on all sides. 
The earlier idea of Black Hills geology was that uplift was due to 
intrusion of the granite, (Newton, Crosby) but it has since been shown 
that the granite, like the schists into which it is intruded, is Algonkian 
and that the uplift took place at the close of the Laramie (Carpenter, 
Irving, a:, b., Jaggar, Darton, a., b., c., Van Hise). In the north- 
ern part of the Hills is a large area where intrusive (Eocene) porphyry, 
contemporaneous with the uplift, forms dikes in the nearly vertical 
schists, and sills and laccoliths in the gently sloping sediments. That 
the later intrusions occur only in the northern hills may be due to the 
better cementing of the southern Algonkian by the earlier granite and 
its accompanying mineralizers. Such pegmatitic granite has not 
hitherto been described from the Deadwood district. 
While members of the Harvard Summer School in 1904, the writers 
had the opportunity of studying, under the direction of Dr. Jaggar, 
a small laccolith in the northern Black Hills, about two miles north- 
east of Deadwood, S. D. During our study of this district two features 
of especial interest were noted, namely, the occurrence of Algonkian 
schist and granite under the domed up Cambrian and the presence of 
numerous bodies of porphyry to the north and northeast of White- 
wood Peak, as shown in Plate 1. 
The laccolith has been described by Jaggar ( p. 217) as follows: — 
“In Whitewood Canyon immediately west of Whitewood Mountain 
is shown a small laccolithic mass of rhyolite which in cross section 
resembles Black Butte in the Judith Mountains, described by Weed 
and Pirsson. The orifice of intrusion appears to have been a fault 
with upthrow on the north, and a second fault of similar character 
appears 1 mile farther south, up the canyon. These faults die out 
within a short distance of the porphyries east and west, and thus appear 
to be genetically associated with the deformation occasioned by in- 
trusion. The laccolith is quite unsymmetrical, the Silurian and 
