CHAP. VI 
SILLS AND LACCOLITES 
87 
111 Colorado, Utah and Arizona the eruptive magma, usually a porphy- 
rite, diorite or quartz-porphyry, has risen in one or more pipes, and has then 
intruded itself laterally between the planes of the sedimentary formations 
which, over the centre of intrusion, have been pushed upward into a vast dome- 
shaped or blister-like elevation. The horizon on which this lateral and 
vertical expansion of the intruded material took place would seem to have 
lain several thousand feet below the surface. It ranges from the Cambrian 
to the Tertiary formations. Subsequent denudation has cut down the up- 
raised mantle of sedimentary layers, and has revealed more or less of the 
igneous rock underneath, which is thus allowed to protrude and to be affected 
by atmospheric erosion. In this way, wdde plains of horizontal or gently 
undulating Secondary and Tertiary strata have been diversified by the 
appearance of cones, detached or in groups, which have become more peaked 
and varied in outline in proportion as their original sedimentary covering 
has been removed from them. Tlie largest of the laccolitic masses in the 
Henry Mountains is about 7000 feet deep and about 4 miles in diameter. 
Less than one-half of the cover of overarching strata has been removed, and 
denudation has cut deeply into the remaining part. 
That the type of structure, so well exhibited among the Henry 
Mountains, has not been more abundantly recognized elsewhere probably 
arises from the fact not that it is rare, but that the conditions for its 
development are seldom so favourable as in W estern America. Obviously 
where stratified rocks have been much disturbed, they cease to furnish 
definite or regular platforms for the reception of eruptive material, and to 
aflbrd convenient datum-lines for estimating what was probably the shape 
of the intruded magma. We may believe that the effect of the propulsion 
of eruptive material is usually to upheave the overlying crust, and thus to 
give rise to a laccolitic form of intrusion. The upheaval relatively to the 
surrounding country will he apt to be practically permanent, the intruded 
body of rock being welded to the surrounding formations, and forming in this 
way a solid and resisting core directly united by piipes or funnels with the 
great magma-reservoir underneath. On the other hand, where the molten 
rock, instead of consolidating underground, has been copiously discharged at 
the surface, its emission must tend townrds the production of cavernous 
spaces within the crust. The falling in of the roofs of such caverns 
will gi\'e rise to shocks of earthquakes. Subsequent uprisings of the 
magma may fill these spaces up, and when the rock has solidified in the 
form of laceolitcs or bosses, it may effectually put an end there to further 
eruptions. 
Some contact metamorphism may be observed along the upper and under 
surfaces of large sills. The rocks over the American laccolites have some- 
times been highly altered. But as' the change is the same in kind as that 
attendant upon Bosses, though generally less in degree, it will be considered 
with these intrusive masses. The problems in terrestrial physics suggested 
by the intrusion of such thick and persi.stent masses of eruptive material as 
those which ^orm sills and laccolites will likewise be discussed in connection 
