IGNEOUS ROCKS. 47 
caused irregularities of structure. Individual strata of sediments 
may be followed almost entirely around the andesite areas. The 
contact with the sediments is commonly steep and the dip of the 
sediments less steep. Less commonly both the dip of the contact 
and of beds is low. The dip of beds and contact varies with depth 
of erosion. Nowhere has erosion exposed the base of the laccoliths, 
though the tops are well stripped. The maximum thickness exposed 
is 1,600 feet. 
The circular outline of the intrusions, the manner in which the 
sediments encircle them and rest against and upon them, and the 
texture of the andesite favor the view that they are laccoliths (PL II, 
pocket). With their relations to the sedimentary rocks, the only 
alternative explanation would be that of a batholitic intrusion, and 
against this stands their texture and the lack, in the surrounding 
sediments, of the schistosity that often characterizes a plutonic 
contact. It is with some confidence, therefore, that the andesite 
masses are concluded to be laccoliths, notwithstanding the lack of 
direct observation of shape of the lower parts. 
According to Gilbert's study of the laccoliths of the Henry Moun- 
tains , a the pressure of injection remaining constant, the limital area 
of a laccolith is a direct function of its depth beneath the surface. The 
limital area is greater when the depth is greater, and less when the 
depth is less. A laccolith with the diameter of the Iron Mountain 
laccolith would require a minimum of 7,000 feet of covering. A 
possible covering of about 4,500 feet can be measured on the 
eroded edges of the surrounding sediments. If Gilbert's conclu- 
sion be a sound one, it may be inferred that in the Iron Springs dis- 
trict certain sediments have been completely removed by erosion. 
His calculation, however, is based on the assumption that the sedi- 
ments have been elastic and free to slide one over the other during 
the intrusion. If, on the other hand, the overlying sediments have 
any considerable strength, developing resistance beyond that afforded 
by the weight, the size of the laccolith would be proportionally 
increased. Such resistance may well be possessed by the Homestake 
limestone, which in all but a few places immediately overlies the 
laccolith, and the large size of the Iron Springs laccoliths may be 
due to this cause rather than to. any greater depth of covering than 
can be measured on the sediments now present in the area. 
A similar conclusion as to the factors determining the horizon of 
the laccoliths is reached from a consideration of densities. The 
density of the andesite is 2.65. The mean density of the rocks known 
to overlie it is about 2.54. Gilbert argues for the Henry Mountains 
that the laccoliths came to rest in such a position that their density 
— «. . 
a Gilbert, G. K., Report on the geology of the Henry Mountains: U.S. Geog. and Geol. Surv. 
Rocky Mtn. Region, 2d ed., 1880, p. 84. 
