academy o F sciences] POWELL 's SURVEY 79 



zontal strata beneath the heavy igneous masses, whereby to his astonishment their blisterlike 

 structures were demonstrated. On this point he wrote: 



If the structure of the mountains be as novel to the reader as it was to the writer, and if it be as strongly 

 opposed to his preconception of the manner in which igneous mountains are constituted, he may well question 

 the conclusions in regard to it while they are unsustained by proof. I can only beg him to suspend his judgment 

 until the whole case shall have been presented (p. 18). 



This presentation he then proceeds to make, not by narrating his personal experiences in 

 the field, a plan which serves well enough in popular exposition, but, as he was addressing 

 professional geologists, by at once setting forth the general conclusion that he finally reached 

 as to the structure of laccoliths, and then marshaling the facts in systematic order as a means 

 of showing how fully his general conclusion would take account of them, great as their variety 

 appeared, and how necessary the adoption of the conclusion therefore was; or in his own words: 



The preliminary explanation of the type structure furnishes a complement of categories and terms by the 

 aid of which the description of the details of observation, essentially tedious, is greatly abbreviated. 



The facts were marshaled in a very convincing procession: First, the up-domed strata, 

 not yet sufficiently worn away to reveal the inferred cistern of igneous rock beneath; then a 

 series of more and more unroofed igneous masses ; and finally those masses, which are not only 

 completely stripped of their former cover, but are so far undermined around their borders as to 

 give clear view of the undisturbed strata underlying them. 



The erosion of the mountains has given the utmost variety of exposure to the laccolites. In one place 

 are seen only arching strata; in another, arching strata crossed by a few dikes; in another, arching strata 

 filled by a net-work of dikes and sheets. Elsewhere a portion of the laccolite itself is bared, or one side is 

 removed so as to exhibit a natural section. Here the sedimentary cover has all been removed, and the laccolite 

 stands free, with its original form; there the hard trachyte itself has bee,n attacked by the elements and its form 

 is changed. Somewhere, perhaps, the laccolite has been destroyed and only a dike remains to mark the fissure 

 through which it was injected (21). 



It is thus evident that, although the general structure of the mountains had been quickly 

 discovered by a few comprehensive views from mountain tops, their detailed structure had been 

 learned by patient and close-range observation. 



Following the general statement, nine examples of domed strata are described in detail as 

 more or less eroded, but not sufficiently so to reveal any igneous rocks; five, of domed strata 

 intruded by dikes and sheets ; eight, of partly revealed igneous cores ; five, of cores well revealed 

 on one side at least and there showing undisturbed strata beneath; two, of fully exposed lacco- 

 lithic cores; and seven of partly demolished laccoliths; the last nine, like the preceding five, 

 presumably exhibiting the underlying horizontal strata, although this significant item is not 

 specified in the summarized statement. No example, however, was found in which the demoli- 

 tion of the cistern mass was so far advanced as to reveal the expected feeding dike beneath it. 

 It is then held that the type structure "accords with all the facts that have been observed and 

 unites them into a consistent whole" (54). The existence of laccoliths was thus established, 

 and at the same time the mountains that they now form were shown to result from intrusion 

 and doming or up-arching of various dimensions, followed by erosion of various degrees. 



There can, therefore, be no question that Gilbert had abundant evidence for his conclusions 

 as to the structure of laccoliths; and, structurally considered, they might be defined as masses of 

 instrusive igneous rock, supplied through unseen chimneys beneath, roughly circular in plan, 

 and having a domelike form, with gently curved top, steep-sloping sides, and flat floors, between 

 undisturbed strata below and up-arched strata above. Even Mount Ellsworth, a large domed 

 structure, one of the southernmost and least denuded of the group, in which overhead dikes are 

 seen but no central laccolith is revealed, illustrates the immensity of the erosion involved in the 

 production of the existing forms; for it is believed to have been originally covered with domed 

 strata more than a mile in thickness. Regarding this fine example, Gilbert reports with charac- 

 teristic frankness that although the laccolith itself is not exposed to view, he is convinced that 

 it exists ; 



that the visible arching strata envelop it, that the visible forest of dikes join it, and that the visible faulted 

 blocks of the upper mountain achieved their displacement while floated by the still liquid lava. The proof, how- 



