544 SIDNEY PAIGE 
strata once passed unbroken over a steep dome. In fact, Jaggar 
refers to this mountain as a type of steep-dome laccolith. 
The facts presented above (Fig. 1) show that this is not strictly 
the case. The intrusive evidently cross-cuts much of the Cambrian 
section at the northern end of the porphyry, while at the southern 
end even the Whitewood, Englewood, and Pahasapa limestones are 
transected. The presence of a small mass of Cambrian sandstone 
lying against the porphyry on the one hand and the Minnelusa sand- 
stone on the other hand adds weight to this idea that the magma 
violently forced its way through this portion of the overlying strata 
much as a punch might perforate plastic material. It is probable 
that such evidence of violent dislocation decidedly influenced the 
conclusions of Russell when he termed these masses igneous plugs. 
Considerations which take into account, however, the configura- 
tion of a great number of intrusive bodies in this region, and an 
examination of the surface structure in the region about Crow 
Peak leave little ground for supposing that this Crow Peak uplift 
is different in its broad essentials from these other laccolithic 
bodies, and it only remains, therefore, to discuss and offer a plausible 
explanation for its difference in detail, that is, the evidently violent 
rupture of its summit and the particular curve which the dips of the 
sedimentary cover would indicate that the magma possesses beneath 
its covering strata. 
It is of interest first to call attention to a fundamental difference 
in the character of the curve on the upper surface or flanks of the 
Henry Mountains laccoliths and that on many of the Judith 
Mountain laccoliths, Montana. Pirsson' has noted this difference 
but does not comment upon it at length. The ideal cross-section 
of the laccoliths of Mount Holmes? (see Fig. 2) as drawn by Gilbert 
after a careful study of the field may be compared with a typical 
cross-section as drawn by Pirsson3 (see Fig. 3) of Judith Mountain. 
While the cross-section of the upper surface as pictured by Gilbert 
is everywhere convex upward, that pictured by Pirsson is locally 
very straight or slightly concave upward. 
1 Op. cit., p. 580. 
2 Opposite p. 28. 
3 Op. cit., Plate LX XXII, section A.—A. 
