82 GROVE KARL GILBERT— DAVIS [MH " ora8 [vouxx£ 



for assigning the Henry Mountains masses an intrusive instead of an eruptive origin, constrained 

 himself, though still incredulous and unconvinced, to announce that interpretation with the 

 reservation that Gilbert's observations needed verification. 3 



More critically pertinent were the doubts expressed by the English geologist, Green, in 

 Nature; for this reviewer, apparently familiar with large igneous intrusions that had broken 

 and upheaved the strata through which they had risen and that had been explained as huge 

 columnar, or blunt conelike masses extending with full or increasing diameter indefinitely 

 downward, was skeptical as to the sufficiency of the reasons offered for the existence of a lim- 

 iting undersurface at the base of the laccoliths. He wrote, "in no captious spirit," yet in some 

 incredulity, questioning the "evidence by which the existence of these peculiarly shaped 

 bodies of intrusive rock is supported. Mr. Gilbert has evidently seen enough to satisfy him- 

 self on this point, and we are quite willing to put every confidence in the statements of so 

 accurate and skilful an observer; at the same time we cannot help feeling some regret that he 

 has not been a little more explicit in his description of the sections which lay open the charac- 

 teristic form of the laccolite. The horizontal base and the undisturbed state of the underlying 

 strata are the first points on which we wish to be thoroughly assured. . . . The views of the 

 Marvine laccolite in Figs. 43 and 44, if we understand them aright, do not seem to be conclusive 

 on the point of the horizontal base; but the evidence would have been more convincing if these 

 plates had been explained at greater length in the text." 4 



It is true that Gilbert did not take pains to distinguish the igneous masses of the Henry 

 Mountains from deep columnar intrusions, although, as is shown above, he distinguished them 

 carefully enough from buried volcanoes. The evident reason for disregarding columnar intru- 

 sions is that, while his published statements concerning the occurrence of horizontal strata 

 underlying the laccoliths are as a rule not so emphatic or so conspicuous as to arrest the attention 

 of a hurried reader, his field studies had so clearly discovered the presence of such strata, that 

 the possibility of the downward extension of the igneous masses, except in inferred feeding 

 dikes, was for him completely excluded. The field notebooks for the season of 1876 leave no 

 doubt upon tins point. Some 25 pages of notes witten on the rainy October 12 were filled 

 with a structural summary which included the following explicit statements : 



In the NE Butte [Jukes ?] none of the curved [overlying] strata are visible, the whole summit is trap. But 

 the level beds below are visible on nearly every side. ... In 249 [Marvine?] every element of the type, except 

 the supply dike or chimney, is clearly visible. At one end the upper strata complete the arch; at the other they 

 have been completely removed and the trap can be seen resting on level strata. 



Three other examples of visible underlying beds are also instanced. A week earlier the 

 general relation of the intrusions had been clearly generalized in the case of two mountains of 

 complex structure: 



The general structure of Ellen and Pennell [in the northern and central parts of the group] is a system of 

 bulge intrusions of trap compiled as irregularly as the secondary cones of a volcano. Successive jets of trap 

 finding passage at diverse points ceased their upward movement at points equally diverse & spread in lenticular 

 form, lifting the superior strata. 



And here the occurrence of undisturbed underlying strata is only implied instead of being 

 explicitly stated. 



Explicit statements on this point in the published report are brief, as follows: As to the 

 Steward laccolith, east of Hillers, " at one end it is bared quite to the base, and the sandstone 

 floor on which it rests is brought to light" (33). Beneath the neighboring Howell laccolith, 

 " the underlying strata, locally hardened to sandstone, lie level; the overlying curve downward to 

 join them" (34, 35). The Sentinel Butte laccolith, north of Pennell, is "sapped by the yielding 

 of its soft foundation" (38). Laccolith E "rests upon the Tununk sandstone" (41). The 

 Shoulder laccolith overlies the Geikie, and a bed of "conglomerate runs under the one and over 

 the other and separates them" (42). Under the Jukes laccolith, the northeasternmost of the 

 group, "are five hundred feet of softer rock which constitutes its pedestal" (46). These state- 



» Erdgeschichte, 1887, i, 180. 



' A. H. G[reen). Nature, xii, 1879, 177-179. 



