548 SIDNEY PAIGE 
is quite possible that breaks will occur and their configuration will 
be more or less circular—the fault surface more or less cylindrical. 
It is this end which is believed to have been reached at Crow 
Peak. In this connection attention is again directed to the horizon 
against which much of the intrusion now rests, viz., soft shale of 
the Deadwood formation, and to the small fragment of sandstone 
of the Deadwood formation faulted against Minnelusa sandstone 
at the southern end of the intrusive mass. The shale of the Dead- 
wood formation would form an ideal locus for such a break as is 
hypothesized after the beds had assumed high dips; and the block 
of sandstone of the Deadwood formation may well represent ma- 
terial dragged up along a fault plane. 
Iddings says * 
But when vertical displacement with faulting is one of the chief character- 
istics of the intrusion a distinction from normal laccolithic intrusion should be 
recognized. In the extreme this would result in the forcing upward of a more 
or less circular cone or cylinder of rocks which might be driven out at the surface 
of the earth, not necessarily in a coherent condition, or might be arrested at any 
stage of such extrusion and so might terminate in a dome of strata resembling 
the dome over a laccolith. By this mode of intrusion the vertical dimension 
of the intruded mass becomes still greater as compared with the lateral dimen- 
sions, so that its shape is more that of a plug or core. 
Such an intruded plug of igneous rock may be termed a bysmalith. There 
is then a transition from a flat intrusive sheet to a laccolith with lenticular 
form and from this to a bysmalith with much greater depth and considerable 
vertical displacement. 
Jaggar in commenting on the description of Mount Holmes by 
Iddings, says:?. “‘The sections and the text indicate that the mass 
described resembles the steep-sided laccoliths of the Black Hills and 
that it breaks across strata in the manner of a stock.” 
Before summing up, specific reference should be made to the 
work of Pirsson. He says: 
It is of interest to note that the convexity of a laccolith is not a necessary 
function of its depth but of the viscosity of the lava in relation to the other 
factors; hence laccoliths of various shapes and sizes may be found in the same 
horizon. We have seen this, from the present work, as occurring in the Judith 
Mountains. 
«J. P. Iddings, ‘“‘Bysmaliths,” Jour. Geol., VI (1898), 705. 
2 Op. cit., p. 289. 3 Op. cit., pp. 585-86. 
