256 PROCEEDINGS OF SECTION C. 
The true key to this hitherto unsolved problem will pro- 
bably be found in the results of the study of similar 
phenomena in the southern portion of the Rocky Mountain 
region in North America. Here the laccolite theory was 
promulgated in 1877 by G. K. Gilbert, of the United States 
Geological Survey, as the only possible explanation of the 
existence of mountain masses of igneous rock. which must 
have cooled at a great depth below the surface. The history 
and development of this theory, and its practical application, 
are exhaustively treated by Whitman Cross in his paper, 
“On the Laccolitic Mountain Groups of Colorado, Utah, 
and Arizona.”(‘1) The “laccolite,’ after all, is only a 
“sill” on a gigantic scale, and the term is restricted in this 
paper to a mass of igneous rock which, ascending as a molten 
magma from unknown depths, has spread out and formed 
a chamber for itself at a certain horizon, lifting up the load 
of sediments above it. In some of the instances cited, the 
uplifted sediments are seen to arch over the porphyrite 
which forms the mass of the mountain; in others, the 
igneous rock has been bared to a depth of thousands of feet 
by their removal. As to other relations of the igneous 
and sedimentary rocks, there appears to be no general rule. 
Where the latter are still seen resting on the laccolite they 
are found to be always more or less altered, while che cor- 
responding strata near the base of the mountain often show 
little or no trace of effects of intrusion. These, too, are 
sometimes seen to dip away from the central mass at a steep 
angle, while elsewhere they abut against it without any sign 
of disturbance. 
The Henry Mountains in Southern Utah were thus 
described by Gilbert:—-‘‘ They are not a range, and have 
no trend; they are simply a group of individual mountains, 
separated by low passes, and arranged without discernible 
system. The highest are about 5000 feet above the plateau 
at their base, and 11,000 feet above the ocean.” Of the 
Mt. Hillers laccolite the report says—‘Its depth is about 
7000 feet, and its diameters 4 miles and 33? miles. Its 
volume is about ten cubic miles. The upper half constitutes 
the mountain, the lower half the mountain’s deep-laid foun- 
dation. Less than one-half has been stripped of its cover 
of overarching strata; the remainder is still mantled and 
shielded by sedimentary beds . . . The uncovered part 
is scored so deeply that not less than 1000 feet of its mass 
are shown in section.” Of the West Elk Mountains, in 
(') Fourteenth Annual Report of the Director of the United 
States: Geological Survey, 1892-3. 
