ACADEMY OP SCIENCES] POWELL'S SURVEY 93 



higher value on cohesion than on density in the explanation of such occurrences, but even then 

 he might still have very reasonably held to his original explanation for the Henry Mountains 

 laccoliths. 



True, it has been proved that the erupted lavas in a certain district of the Yellowstone Park 

 were supplied by the same magma that fed a number of underlying dikes and sheets, now laid 

 bare by later erosion; but the sheets are not especially laccolithic in form, and moreover the 

 disturbed structure of that region makes it possible that diastrophic forces as well as the pressure 

 of the rising magma had to do with the opening of underground fissures for the deep-seated 

 intrusions; and in such case these phenomena would fall into a different category from that 

 which includes the typical laccoliths of the undisturbed Henry Mountains district. It is true 

 also that Gilbert's explanation of his laccoliths in terms of the hydrostatic law requires, as he 

 candidly stated, the assumption that the present relation of densities in laccolithic and volcanic 

 rocks should have been reversed when those rocks were molten and before the volcanic rocks 

 in particular had lost a large share of their originally included gases; and this assumption has 

 not yet been verified. But if a thinker like Gdbert found value in the assumption, that is a 

 good and sufficient reason for presenting it as a part of his theory of laccoliths; yet, such pres- 

 entation is rarely if ever found. 



EFFECT OF MAGMATIC VISCOSITY 



As to the second point — the control of the diameter and domelike form of laccoliths — Dana's 

 explanation in his review is that, as the lighter magmas are the least fusible, viscosity becomes 

 dominant in determining the form that they assume ; they are " easily chilled and thicken greatly 

 in the upper part of narrow fissures or of volcanic conduits, and it is for this reason that they 

 have so often made steep sided domes over subaerial vents." This point is somewhat more fully 

 treated in the latest edition of his Manual : 



The thickness [of intrusions] depends somewhat on the fusibility of the rock, the more fusible kinds making 

 extended masses or sheets, and the less fusible producing thicker and more bulging forms. . . . The breadth of 

 the [laccolithic] mass is consequently only three to seven times greater than the height. 5 



Here the implication is clearly made that the magma of a laccolith begins to thicken ver- 

 tically as soon as it turns to spread laterally, as if its viscosity were so great that its upward 

 pressure could not be readdy turned into a lateral or horizontal pressure. But if a domelike 

 form is thus early assumed, the sharp bending of the domed strata could be accomplished only 

 at a great mechanical disadvantage, for in such case the strata above the dome would have to 

 be strongly up-arched whde the laccolith is still small ; moreover, the strata over the center of 

 a completed laccolith, which has a nearly flat-topped dome, are very little arched. Hence, on 

 the assumption of a viscous magma which begins to thicken as it begins to spread, the strata 

 over its center must be first up-arched and afterwards flattened again. Such a doing and undoing 

 is conceivable, but it does not seem probable. 



Furthermore, the assumption of a viscous magma overlooks or sets aside the evidence that 

 convinced Gdbert of the effective fluidity of the magma; more serious still, this explanation 

 neglects the observed facts of the Henry Mountains which indicate that full-sized thin sheets 

 were formed before their thickening into domelike masses was begun; and most unfortunately 

 of all, it takes no adequate account of the mechanics of doming, and nowhere recognizes the 

 pertinent principles that " the rigidity or strength of a body is measured by the square of its 

 linear dimensions, whde its weight is measured by the cube," and that "a small laccolite [of 

 fluid lava] can not lift its small cover, but a large laccolite can lift its correspondingly large 

 cover" (97). Gdbert alone has considered these essential elements of the laccolith problem. 



Fortunately, however, Dana made a nearer approach to GUbert's view on the next page of 

 his Manual: "As Mr. Gdbert states, the intrusion of the lava laterally into a chamber widened 

 the area of pressure, and thus enabled it, on the principle of the hydrostatic press, to accomplish 

 the lift by very slow steps of progress" ; but even this does not recognize the mechanical advantage 

 that a sheet of large diameter has over a sheet of smaU diameter beneath a given thickness of 

 cover in raising the overlying strata and thickening itself into a domelike mass; nor is recognition 

 given to the remarkable confirmation for Gdbert's theory that is found in the increase of the 

 diameters of laccoliths with depth. 



. , im~— 



I Manual of Geology, 4th ed., 1895, 301. 



