academy of sconces] POWELL'S SURVEY 95 



The station of the laccolite being decided [namely, when lavas . . . reach the zone in which there is 

 the least hydrostatic resistance to their accumulation], the first step in its formation is the intrusion along a 

 parting of strata, of a thin sheet of lava, which spreads until it has an area adequate, on the principle of the 

 hydrostatic press, to the deformation of the covering strata. ... So soon as the lava can up-arch the strata 

 it does so, and the sheet becomes a laccolite (95). 



Twenty years later, Gilbert quoted another writer's view that viscosity is " an essential 

 condition in the production of thick intrusive lenses," but added that "this theory encounters 

 serious difficulty." 8 This appears to be the most explicit reference that he made to viscosity. 

 In his judgment it did not seem to deserve consideration. 



It would, indeed, be extremely difficult to imagine that the upward flexing of the strata 

 which cover the thin Howell laccolith began, as the assumption of a viscous magma demands, 

 when the diameter of the spreading igneous mass was small, and that the flexing was then 

 continued radially outward as the diameter increased; for that process of deformation would 

 involve a progressive unflexing of the first-flexed central strata, in order that they should now 

 lie flat on the upper surface of the intruded mass. Hence, thin as this laccolith is, its initial 

 intrusion must have been thinner still; and the thickening to the present modest measure of 

 50 feet, by which the upflexing of the marginal strata was caused, could not have been begun 

 until the present diameter of the laccolith was reached. But it is not this exceptionally thin 

 laccolith alone that furnishes evidence for the initial intrusion of thin sheets. Additional 

 evidence is given by certain other laccoliths which "appear to be built up of distinct [igneous] 

 layers." Even the thin Howell laccolith shows two such layers. 



The Peale. exhibits three layers with uneven partings of shale. The Sentinel shows two without visible 

 interval. . . . The Pennell has a banded appearance but was not closely examined. ... It is probable that 

 all the larger laccolites are composite, having been built up by the accession of a number of distinct intrusions (55) . 



Hence, as far as the Henry Mountains laccoliths are concerned, even if the magma were 

 imperfectly fluid, it was fluid enough to spread in comparatively thin sheets of a considerable 

 diameter before it began to thicken into domelike forms, either by its own intumescence or by 

 the addition of new sheets. Viscosity may play a part in determining the form of other intrusive 

 masses, especially when it is of a much higher degree than it can have been in the Henry Moun- 

 tains laccoliths; and extreme fluidity as well as a relatively high density may have had to do 

 with the great horizontal extension of certain thin intrusive sheets, such as the whin sill of 

 northern England; but viscosity does not appear to have determined the doming of typical 

 laccoliths. Gilbert evidently had good reasons for his conclusions in this problem. It must 

 therefore be urged that whatever explanations may be offered for intrusions of various irregular 

 forms in divers crustal structures, the explanation that Gilbert presented in the middle chapters 

 of his report for the intrusion of the typical laccoliths of the Henry Mountains into the undis- 

 turbed strata of southeastern Utah takes fuller account of all attendant conditions and is more 

 critically and convincingly analyzed than any of its successors. The explanation may not be 

 right in every particidar, but it has not yet been shown to be wrong for the laccoliths that it 

 treated. It may be suggested that rate of intrusion, a factor to which Gilbert gave little atten- 

 tion, may nevertheless have been of importance in determining the form assumed by the intruded 

 magma. A very slow intrusion might gain a horizontal dimension much greater than the 

 limited diameter of Gilbert's theory, and a very rapid intrusion might break the covering strata 

 and enter therein irregularly. 



If the foregoing pages appear at first reading to be devoted more largely to the discussion 

 of a peculiar geological problem than is appropriate in a biographical memoir, let it be under- 

 stood that they have really been devoted to an exposition of the care with which the analysis 

 of a new problem was conducted by an eminent geologist. Whatever value his analytical 

 explanation of laccoliths may have in making known the true nature of the Henry Mountains, 

 an understanding of the explanation is indispensable if one would know the true nature of its 

 inventor. Indeed, the middle chapter of the Henry Mountains report possesses, to a degree 

 that is almost or quite without rival, that clearness of treatment which became a pronounced 

 characteristic of all Gilbert's later papers, and which gave him the rank of a leader among 



« Journal of Geology, IV, 1886, 821. 



