42 USIKAIBIE (Cn IRS SVBIEIC 
that they cannot be referred to the class of intrusions that has its 
type in the Henry Mountains. 
What precise mode of origin Newton had in mind when he 
compared these intrusions to bubbles ina viscid lava mass, is not 
clear. As the nature of laccolites was not understood at the time 
of his exploration, a comparison with that form of intrusion could 
not be made. 
That the magma composing the hills described in this paper 
was cooled slowly at a considerable distance below the surface, 
and consequently under great pressure, is indicated by their geo- 
logical associations and is proven also by the character of their 
crystalline structure. They are coarse grained or porphyritic, 
instead of being glassy and imperfectly crystallized. In no 
instance has the rock assumed the form of obsidian, pitchstone, 
pearlite, etc., or been expanded into scoria and pumice, as one 
would expect had the magma cooled at the surface. 
None of the plutonic plugs examined by me are associated 
with dikes or faults. In fact dikes appear to be wanting in the 
Black Hills region, since they do not seem to be mentioned by 
any of the geologists who have written concerning it. It is 
reasonable to suppose that the magma which rose from below 
and formed the plug-lhke intrusions described above, found its 
way through fissures in the lower series of stratified rock, but 
proof that this was the case is wanting. How the stratified beds 
below the domes that covered the plugs were displaced, or per- 
haps fused, so as to furnish room for the passage of the intruded 
material, is not clear. 
A comparison of the structure of the Sun Dance hills, Inyan 
Kara, etc., which are from one to two or three miles in diameter, 
with the structure of the dome that once covered Warren Peak, 
and had a diameter in one direction of two to three miles, in 
another direction of about eight miles and a height of two or 
three thousand feet; and still again with the structure of the 
vastly greater dome from which the Black Hills, as we know 
them, were sculptured, brings out striking similarities. Had no 
erosion taken place since the Black Hills uplift began, as is shown 
