ON THE NATURE OF IGNEOUS INTRUSIONS 185 
and general structure of the uplift are concerned, and shared in 
the disturbance that elevated them. Whether the core of 
intruded granite answers to the plutonic plugs forming the cores 
of the smaller domes or has itself been elevated by an intrusion 
of molten rocks beneath, will be considered in advance. 
Under this view of the origin of the Black Hills dome, the 
supposed intrusion of molten rock took place deep below the 
surface, and on account of the vast weight to be lifted, and prob- 
ably also by reason of its viscosity, did not expand into a broad 
sheet, but as in the case of true laccolites, and plutonic plugs, 
produced a local elevation. The deeper the horizon at which 
the intruded rock expanded, the broader would be the dome 
formed above it. The outline of the dome raised on the surface 
would depend on the form of the cistern of molten rock beneath, 
and perhaps on the dip of stratified beds before the intrusion 
occurred. If the subterranean cistern was supplied through an 
extended fissure, an elongated dome ora ridge-like elevation 
at the surface would result. The domes produced by the injec- 
tion of plutonic plugs, as already stated, are nearly circular, 
but the major axis of the Black Hills dome is double the length 
of the minor axis. The form that the elongated Black Hills 
dome would present, had there been no erosion, is shown on a 
contour map by Newton and Gilbert.’ 
The influence of dip on the form of an uplift produced in the 
manner above considered, is probably not important, as it is 
doubtful if such deformation occurs in rocks that are not essen- 
tially horizontal. 
Mountains of the same type as the Black Hills but in which 
the difference between the horizontal axes is greater, occur in 
the same general region. 
The mountains of Wyoming and Colorado.—The Big Horn 
Mountains, Wyoming, are of the same type as the Black Hills 
uplift, but the dome from which they were sculptured was more 
elongated than is the case of the Black Hills dome. The trend 
of the range is approximately northwest and southeast, but is 
"Report on the Geology of the Black Hills, plate opposite page 208. 
