542 SIDNEY PAIGE 
important factor in determining the form of certain laccoliths— 
increasing viscosity during intrusion—is believed to have been 
carried to its ultimate results. Since this phase of the discussion 
has not been presented before in a connected way, the writer hopes 
the following notes may be of interest. 
Near the northwest corner of Spearfish quadrangle in the 
Black Hills of South Dakota, Crow Peak reaches an elevation of 
5,785 ft. This peak is the crowning point of a rugged, sharply 
dissected, isolated mountain about 13 miles long and a mile wide, 
its larger dimensiori lying north-northwest. 
The mountain owes its presence to an intrusive mass of quartz 
monzonite porphyry. This porphyry outcrops as an elliptical 
area with its longer diameter (about a mile) trending north-north- 
west. Where the porphyry breaks through the overlying sedimen- 
tary rocks the strata have been bent sharply upward and steep dips 
(up to go”) are found at all points on the periphery of the intrusion. 
These dips become much gentler a short distance from the intrusion 
and within a half-mile or less the beds have assumed the nearly 
horizontal attitude prevalent over this region. 
There is strong presumptive evidence, however, that the igneous 
mass has a tongue-like extension northwestward beneath the sedi- 
mentary rocks, for the axis of a clearly defined anticline extends in 
this direction for several miles from the base of the mountain. 
Such an extension is not indicated to the southward. 
The sedimentary rocks involved at this place rest upon a pre- 
Cambrian basement and are as follows: Cambrian quartzite, 
sandstone, and shale (Deadwood formation), almost invariably a 
thin shale at the top above a thin quartzite, 400 ft. =; Ordovician 
limestone (Whitewood), 80 ft. + ; Carboniferous: shaly Englewood 
limestone at base 30 ft. +; overlain by Pahasapa limestone, 550 ~ 
ft. =; succeeded by Minnelusa sandstone, 400 ft. =; in all 1,460 
Hig) SS 
A conception of the mechanics of the intrusion must be in large 
part based upon speculation, for data are not at hand even to 
definitely establish the underground form of this igneous mass. As 
may be seen on the map (see Fig. 1), the northern half of the 
intrusion cuts as high in the sedimentary beds as the base of the 
