92 GROVE KARL GILBERT— DAVIS [MEM0IR ^v A ™xt 



tion are therefore still debatable. So long as the debate continues Gilbert's theory, which is 

 much more penetratingly argued than any other yetpubhshed, merits fuller recognition than it 

 has received. Three elements of his theory in particular deserve emphasis; the first concerns 

 the value of the hydrostatic law as controlling the level at which a rising magma ceases to 

 continue its ascent and begins to spread laterally; the second concerns the mechanical controls 

 by which the laterally spreading magma is limited in diameter and led to assume a domelike 

 form; the third concerns the influence that the depth of a laccolithic intrusion exerts upon its 

 diameter. As two of these elements of the theory received a certain measure of consideration 

 and yet were rejected in a critical review of Gilbert's report by a leading American geologist, 

 attention may be called to the arguments there adduced. 



dana's alternative theory 



Shortly after the publication of Gilbert 's Henry Mountains report, Dana reviewed it and 

 proposed an alternative explanation for laccoliths — an alternative that was favorably quoted 

 by another writer on Rocky Mountain laccoliths 14 years later — as follows: 



With so powerful a forced movement in the lavas as the facts, if they are rightly interpreted, show to have 

 existed, no other cause could be needed for a flow to the surface in the case of an open channel, or for a flow to 

 any level in the strata at which a [vertical] fissure might terminate, and this is true whether the lava be light or 

 heavy. 4 



This is unfortunately only a rough and ready method of settling the problem at issue, as 

 compared with the deliberate analysis that characterizes Gilbert's discussion. It leaves the 

 main question unsettled; namely, if an ascending magma has the force to act as it chooses, 

 how will it choose to act ? Indeed, Dana 's solution is based on the tacit assumption that rock 

 cohesion overcomes the hydrostatic law, and Gilbert 's reasons for adopting a contrary opinion 

 are not adequately presented either in the review cited or in the later editions of the 

 reviewer's Manual of Geology. Yet if Gilbert's reasons for his opinion are valid, it would 

 follow that even if a column of somewhat dense laccolithic magma rose in an open fissure almost 

 to the surface of the earth's crust, it- would still be easier for the magma to wedge its way later- 

 ally between slightly coherent strata at a hydrostatically determined depth than to overflow 

 at the surface. 



It is, of course, possible that Gilbert may have underrated the value of rock cohesion; 

 and that a heavy magma ascending through a vertical fissure in a less dense but very solid 

 granite might have to rise all the way to the surface instead of spreading laterally at a hydro- 

 statically determined depth; or, on the other hand, that a very light magma might, instead of 

 rising to the surface, spread laterally into a subterranean domelike cavity that was opened 

 for it in very dense rocks by diastrophic forces which did not at the same time open a higher 

 fissure; but until actual examples are found that appear to exemplify one or the other of these 

 possibilities, it seems reasonable at least to announce the well analyzed reasons which led 

 Gilbert to place a higher value on the hydrostatic law than on rock cohesion in his explanation 

 of the actual laccoliths and volcanoes of the Colorado plateaus. 



In the meantime, it should be remembered that Gilbert himself did not by any means 

 overlook the possibilities just suggested regarding the dominance of cohesion; for after briefly 

 examining the relative values of densities and of penetrability, he concluded: "Since the con- 

 dition of penetrability resides in the solid rock only, and the condition of density pertains to 

 both solid and fluid, either condition might determine laccolites at certain stratigraphic hori- 

 zons, while the latter only could discriminate certain lavas as intrusive and others as extrusive" 

 (74). But inasmuch as he found that the lavas of the plateau region are, as a matter of fact, 

 sharply discriminated between intrusions and extrusions, he was "led to conclude that the 

 conditions which determined the results of igneous activity [for that region] were the relative 

 densities of the intruding lavas and of the invaded strata; and that the fulfillment of the general 

 law of hydrostatics was not materially modified by the rigidity and cohesion of the strata" 

 (74, 75). Evidently enough, if he had found examples elsewhere in which relatively light 

 intrusive rocks formed laccolithic masses in relatively dense strata, he would have placed a 



' Amer. Journ. Sci., iii, 1880, 24. 



