546 SIDNEY PAIGE 
suddenly to lift the cylindrical mass of rock above it. Pirsson 
reaches the conclusion that this magma was introduced rapidly, 
but on other grounds! He says: ‘‘The occurrence of ball- 
like masses in the upper crust of the laccolith seems to show that 
the filling took place with considerable rapidity.” Ii now it be 
conceived that magma was introduced more slowly or that its 
viscosity was greater, thereby interfering with the operation of the 
law of hydrostatic pressure, the factor of marginal cooling with 
concomitant increasing viscosity becomes a factor of importance. 
U 7 <7 Om NRA Wed ied a bce tA 
IAIN NUNN US NICO GSIRTATT NTS MTD WINTON lone TIRES IIR EAN 
Fic. 4.—Cross-section of Shonkin Sag laccolith (after Pirsson) 
For just as much as the law of hydrostatic pressure is prevented 
from acting or forced to act more slowly, just so much will there be 
unequal distribution of upward pressure. The region of greatest 
pressure will be where the magma is most fluid, i.e., directly over 
the source of supply, while from this region outward, decreasing 
pressure will be exerted on the roof. The series of diagrams (Figs. 
5-9) illustrate what might take place during intrusion under such 
conditions. ‘The outer border congealing first, the area of perfectly 
transmitted pressure would be reduced, and each successive? 
application of pressure would therefore serve to accentuate the up- 
ward curve of the strata, that is, the curve on the surface of the 
laccolithic flanks in such a system would be more or less concave 
upward. 
At one end of the series, then, we would have the Shonkin Sag 
type with a flat top; under condition of intermediate viscosity the 
type depicted by Gilbert (where the progressive increase of viscos- 
ity was not sufficient to form a curve concave upward, though 
sufficient to prevent a flat roof); and at the other extreme the type 
which the Judith Mountain masses approach and which Crow Peak 
may possess in even greater perfection. 
VOps cit ps 12- 
The process was probably continuous in its effect but successive steps are 
used to express more clearly the idea. 
