PROGRESSIVE INCREASE OF VISCOSITY 545 
The writer in seeking an explanation for the condition at Crow 
Peak, and led by the result of one of Howe’s experiments in arti- 
ficial laccolith building,’ formulated the hypothesis that the form 
of the upper surface of a laccolith might be materially affected by 
the progressive increase in viscosity of the magma during injection. 
ee 
Fic. 2.—Ideal cross-section of the laccolites of Mt. Holmes (after Gilbert) 
It was postulated that due to pressure from beneath, magma 
in fluid condition was introduced at the base of a sub-horizontal 
sedimentary series, and insinuated itself along the basal contact 
forming a thin sill or sheet of roughly circular outline. Such a 
sheet would exert hydrostatic pressure, which if sufficiently great to 
ON a cen a DS NT 
Fic. 3.—Section of laccolith in Judith Mountains (after Pirsson) 
overcome the weight of the overlying strata would initiate the for- 
mation of a dome. If introduced with great rapidity such a sheet 
theoretically might take the form of the Shonkin Sag laccolith’ 
(see Fig. 4) described by Pirsson for this form suggests that the lava, 
being introduced rapidly in a thin sheet, attained eventually an 
area over which the hydrostatic pressure was sufficiently great 
‘Op. cit., experiment III. 
2W.H. Weed and H. V. Pirsson, ‘‘ Geology of the Shonkin Sag and Palisade Butte 
Laccoliths in the Highwood Mountains of Montana,” Am. Jour. Sci., 4th series, 
XII, 1-17. 
