86 GROVE KARL GILBERT— DAVIS [Memoies [ vo A l t, £x a i l 



THE HYDROSTATIC LAW AND ROCK COHESION 



Now disregarding the source of the magma and its propelling force, the question arises: 

 "Why is it that in some cases igneous rocks form volcanoes and in other cases laccolites?" 

 Here successive suppositions are made with a view to simplifjnng the problem. It is first 

 supposed that the solid rocks of the crust have no cohesion to impede either the vertical rise 

 or the lateral spread of the fluid rocks. "The lava will then obey strictly the general law of 

 hydrostatics, and assume the station which will give the lowest possible position to the center 

 of gravity of the strata and the lava combined" (72). This conclusion appears to have been 

 reached in the field, for a notebook record of September 20, 1876, concerning two contiguous 

 intrusive sheets near Mount Pennell implies hydrostatic action in the brief statement: "I call 

 the upper the newer because it was lighter in weight molten than the other cold." A part of 

 the long summary written three weeks later on the rainy 12th of October gives a more general 

 treatment of the same principle. It is evident that careful theoretical reflection recorded in 

 view of the facts must have greatly expedited the preparation of the report in Washington. 



Inquiry is next made into the validity of the hydrostatic law when cohesion is considered. 

 Gilbert recognizes that the strength of the crustal strata must of course modify the behavior 

 of the rising magmas, and then frankly confesses, in a way that immediately begets confidence 

 in his fairness: "I am at a loss to tell in what way it influences the selection by a lava flood of 

 a subaerial or a subterranean bourne" (73). However, he concludes that the hydrostatic law 

 is not wholly abrogated ; it is only modified by cohesion: "Light lavas will still tend to rise 

 higher than heavy, however much the rising of all lavas may be hindered or favored" (74). 

 The problem therefore really turns on whether the relative density of lavas and crust or the 

 penetrability of the crust is the determining factor. "When resistance to penetration is the 

 same in all directions, the relation of densities determines the stopping place of the rising lavas; 

 but when the vertical and lateral resistances are unequal, their relation may be the determining 

 condition." Hence, if penetrability were the controlling factor during an epoch of igneous 

 activity, all kinds of magmas would rise to the surface where the crust is vertically penetrable 

 and would be found together in volcanoes; while where the crust is not thus penetrable but is 

 liftable, all kinds of magmas would fail to reach the surface and would be found associated in 

 laccoliths. But, on the other hand, if density be in control, then certain lighter magmas would 

 usually form volcanic masses and other heavier magmas would form laccoliths. 



In the actual case of the Henry Mountains the difficulty of penetrating and the ease of 

 lifting the strata may have guided the intrusions into two shale horizons, as above noted, but 

 these factors may not alone have been in control. When the constitution of the intrusive and 

 effusive rocks of the plateau province is examined, "we find the entire weight of the evidence 

 in favor of the assumption that conditions of density determine the structure. The coincidence 

 of the laccolitic structure with a certain type of igneous rock is so persistent that we can not 

 doubt that the rock contained in itself the condition which determined its behavior. We are 

 then led to conclude that . . . the fulfillment of the general law of hydrostatics was not materi- 

 ally modified by the rigidity and cohesion of the strata" (75). 



A test for this conclusion is next sought for in the densities of the rocks concerned, and 

 thereupon an apparent contradiction is met; for in surface volcanoes, where the density of the 

 rocks should be low, it is about 3; while in the subsurface laccoliths, where the density should 

 be high, it is only from 2.6 to 2.9. Here, however, Gilbert added: "But in order that the 

 laccolitic structure should have been determined by density, the acidic rock of the laccolites 

 must have been heavier in its molten condition than the more basic rocks of the neighboring 

 volcanoes" (75). Search was therefore made for information as to change of density when a 

 rock passes from the molten to the solid condition, and citation was made of the results of 

 experiments by Bischof, Del esse, and Mallet — much to the dissatisfaction of the last named; 1 

 but the data thus gained were not considered pertinent, because all the experiments involve 

 dry fusion, while "it is generally conceded that the fusion of lavas is hydrothermal." Sub- 



1 Nature, xiii, 1880, 265. 



