ACADEMY OF SCIENCES] POWELL'S SURVEY 91 



statement is made that laccoliths should be " probably not uncommon in denuded volcanic 

 districts." But no reasons are adduced for thus summarily setting aside Gilbert's well-supported 

 induction to the contrary. In another excellent modern work, which usually gives alternative 

 explanations a sufficiently full consideration, laccoliths are treated only in a brief statement under 

 a general account of intrusions: "Sometimes the lava appears to have forced its way into the 

 rocks, and sometimes to have lifted the upper beds and formed great subterranean layers or 

 tumorlike aggregates, called bathyliths and laccoliths." Again, a responsible volume of still 

 later date says nothing of Gilbert's views as to the condition of laccolithic intrusion, but pro- 

 pounds a new explanation for laccolithic domes dependent on the rate of intrusion: "If the 

 supply of material in the formation of an intrusive sheet is more rapid from below than can 

 easily spread laterally, the strata above will be up-arched, as if by a hydrostatic press, and a 

 thick lens of liquid rock will be produced, giving rise on solidification to a laccolith." 



If, instead of examining general textbooks of geology, attention is given to special reports 

 and works on igneous structures, fuller statements concerning laccoliths are found, but none of 

 them do justice to Gilbert's explanation of the intrusions in the Henry Mountains. A report 

 on another laccolithic mountain group, in which many features of the Henry Mountains type 

 are repeated, gives very little attention to the mechanical factors involved in the doming of the 

 covering strata, yet concludes that "the controlling factors are shown to be the viscosity of the 

 magma, the rate of injection, and the load of sediments and their plasticity, when in mass, 

 under stresses"; but as Gilbert's analysis of the behavior of an essentially fluid magma is not 

 explicitly discussed and shown to be wrong, it may be seriously questioned whether viscosity 

 and rate of intrusion are really proved to have the leading values here ascribed to them. 

 Again, a monographic volume on igneous rocks announces: "Among the special students of 

 laccoliths the hypothesis prevails that great viscosity is an essential prerequisite in this mecha- 

 nism" ; yet none of these students is shown to have published an analysis of the problem that 

 in completeness at all approaches Gilbert's, nor have they especially considered the evidence 

 that is found, as will be stated below, in the typical Henry Mountains laccoliths for the high 

 effective fluidity of their magma at the time of its intrusion. Still another volume of mono- 

 graphic quality concludes that the magma of laccoliths is driven upward until it reaches a 

 horizon of easy fissuring, where it spreads laterally; thus incoherence of crustal rocks is tacitly 

 postulated to be of greater importance than hydrostatic pressure, which is not considered; 

 yet if this tacit postulate were true, no volcanoes could be built where the land surface, as in 

 the high plateaus of Utah, consists of a heavy series of horizontal strata in which easily fissured 

 members lie at a considerable depth. It is then added that the form which the spreading magma 

 assumes is primarily a function of its viscosity; thus again the evidence for fluidity and the 

 mechanical factors involved in the doming of overlying strata are left out of the discussion. 



These special reports therefore give little if any better understanding of Gilbert's views 

 than is to be gained from the terser statements of standard textbooks. It is as if, with the 

 recent discovery of a great variety of structures assumed by intrusive rocks, and still more with 

 the recognition of the great variety of rocks found in intrusive structures, the attention of 

 geologists had been directed chiefly to the existing structural complications of laccolithkke 

 masses and to the manifold problems of their magnetic differentiation, and thus diverted from 

 Gilbert's discussion of the conditions under which the simple and typical laccoliths of the 

 Henry Mountains had been formed; and it is furthermore as if, at the same time, a wave of 

 very imperfectly argued opinion in favor of viscosity as a determining factor in the origin of 

 laccohths has been permitted to obscure the evidence against viscosity provided by the Henry 

 Mountains as described hi Gilbert's eport. Possibly other laccolithic magmas were more vis- 

 cous than those of his type examples; possibly a decrease of temperature in the rising magma has 

 made it more viscous in the later than in the earlier stages of a laccolithic intrusion; but these 

 possibilities should not be allowed to blur the evidence which the type examples give for fluidity. 



In any case it would certainly appear from the foregoing citations that no general agree- 

 ment has yet been reached as to the origin of laccoliths, and that the conditions of their forma- 

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