86 
SUBTERRANEAN VOLCANIC ACTION 
BOOK I 
sometimes have failed to complete the formation of an actual volcano. 
Aided by subterranean movements, it might have been potent enough 
to disrupt the lower parts of the terrestrial crust, to propel the molten 
magma into fissures, even to inject it for many miles between the planes 
of stratification, whicli would be lines of least resistance, and yet in default 
of available rents, might have been unable to force its way through the 
upper layers and so reach the surface. Examples of such incompleted 
volcanoes are perhaps to be recognized among solitary sills, which not in- 
frequently present themselves in the geological structure of Britain. But 
the positive decision of this question is almost always frustrated by tlie 
imperfection of the evidence, and the consequent possibility that a connected 
vent may still lie concealed under overlying strata. 
Besides the more usual intrusions of molten material in the form of sheets 
of which the verti(!al thickness bears but a small proportion to the horizontal 
extent, there occur also large and thick cakes of intruded material in which 
the vertical thickness may approach, or perhaps even surpass, the horizontal 
diameter. These dome -shaped or irregular expansions form a connecting 
link between ordinary sills and the bosses to be subsequently described. 
They have received the name of Laccolites from Mr. G. K. Gilbert, who worked 
out this peculiar type of structure in the case of the Henry Mountains in 
southern Utah ’ (Fig. 34). The same type has since been found distributed 
over Arizona and Colorado, and it has been recognized as essentially that of 
many eruptive masses or bosses in all parts of the world. 
In Western America, owing in large measure to the previously un- 
disturbed condition of the sedimentary formations, the relations of the injected 
igneous material to these formations can be satisfactorily ascertained. The 
geological structure of the various isolated laccolites thus clearly presented, 
helps to explain the structure of other intrusive bodies which, having 
been injected among plicated and dislocated rocks, do not so readily admit 
of interpretation. 
' “Geology of the Henry Mountains,” U.S. Geog. and Gcol. Sxwvcy of the Rocky Moun- 
tain Region, 1877. For a review of the whole subject of laccolites in Western America see a 
paper by Mr. Whitman Cross, in the \ith Annual Report of the Director of the U.S. Geological 
Survey, 1892-93 (pub. 1895), p. 157. 
