While there is an obvious relation between most sills and some eruptive centre in their neighbourhood, cases occur in which no trace of any contemporaneous volcano can be found, but where the intrusive sheet remains as the sole evidence of the movements of the subterranean magma. The Great Whin Sill, one of the most extensive intrusive sheets in the British Isles, is an instance of this kind. Though this large mass of injected material can be traced for a distance of about 80 miles, and though the strata beneath and above it are well exposed in innumerable sections, no evidence has yet been detected to show that it was connected with any vent that formed a volcano at the surface (see [vol. ii. p. 2]). The absence of this evidence may, of course, arise from the failure of denudation to uncover the site of the vent, which may possibly still remain buried under the Carboniferous strata that overlie the sill towards the south-east. But it may be due to the non-existence of any such vent. We can quite conceive that volcanic energy should 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, which 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 infrequently present themselves in the geological structure of Britain. But the positive decision of this question is almost always frustrated by the 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 vertical 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[34] ([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.
[34] "Geology of the Henry Mountains," U.S. Geog. and Geol. Survey of the Rocky Mountain Region, 1877. For a review of the whole subject of laccolites in Western America see a paper by Mr. Whitman Cross, in the 14th Annual Report of the Director of the U.S. Geological Survey, 1892-93 (pub. 1895), p. 157.
Fig. 34.—Ideal section of three Laccolites. (After Mr. Gilbert.)
In Western America, owing in large measure to the previously undisturbed 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.
In Colorado, Utah and Arizona the eruptive magma, usually a porphyrite, 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 upraised 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, wide 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. The 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 Western 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 afford 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 be 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 pipes 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 towards the production of cavernous spaces within the crust. The falling in of the roofs of such caverns will give 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 laccolites 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 sometimes 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 persistent masses of eruptive material as those which form sills and laccolites will likewise be discussed in connection with the mechanism of the remaining intrusive masses which have now to be described.