COMPARISON WITH OTHER REGIONS. 233 
at very high angles,* seem more nearly related in general structure, and 
the same may perhaps be said of the extraordinary “plutonic plugs” 
described by I. C. Russell as pushing up the strata into domes in the 
Black hills of Dakota.f Probably some of the bosses of granite described 
in works on geology are of the same character, though many of them 
have a different relationship, pushing or fusing their way through over- 
lying strata without becoming schistose themselves nor doming up the 
beds above. 
The dome of the Black hills, as represented by Russell, { seems to 
come closest to the batholitic domes of western Ontario, though the sec- 
tion across the Black hills, copied from Newton and Gilbert, differs 
ereatly from them in some points. As shown diagrammatically, the rela- 
tively small central plug of granite is surrounded by a wide band of 
vertical schist, on whose edges rests the dome of sedimentary rocks, as 
though there had been two upheavals, separated by a wide interval, 
during which the later sediments of the dome were deposited. The size 
of the Black Hills dome is greater than that of the west Ontario batho- 
lites studied up to the present, and the time of the latest uplift much 
more recent. 
CAUSE OF THESE MounTAIN STRUCTURES 
Gilbert suggests for the laccolites that the ascending flow of molten 
rock rises only until the overlying rock is less dense than itself, when 
the latter is pushed up into a dome, the general law of hydrostatics being 
obeyed.§ 
Whitman Cross, following Dana, does not accept the hydrostatic the- 
ory, thinking that the force which set the lava in motion is sufficient to 
account for the facts.|| 
I. C. Russell suggests *‘ that uplifts welt owe their origin to the intru- 
sion of a molten magma into the rocks beneath them be termed sub- 
tuberant mountains. They may be fancied to originate from the growth 
of a tuber within the earth’s crust.” §/ He thinks that the cooling and 
therefore contracting crust of the earth brings pressure to bear on the 
hotter interior, squeezing upward the molten rock, which may either 
form domes without reaching the surface or come to the surface forming 
volcanoes.** 
*Geol. Survey of Canada, 1882~’83-’84, p. 17, C. 
+ Journal of Geology, vol. iv, no. 1, p. 23 ete. 
t Journal of Geology, vol. iv, no. 2, p. 183, etcetera. I have been unable to obtain a copy of the 
“Geology of the Black Hills” in time for use in preparing this paper. 
2 Geology of the Henry mountains, Washington, 1877, pp. 72 and 95. 
|| Laccolitic mountain groups of Colorado, Utah, and Arizona-Washington, 1895, p, 241, 
q Journal of Geology, vol. iv, no, 2, p, 189, 
** Tbid., pp. 190, 191, 
