PROGRESSIVE INCREASE OF VISCOSITY 549 
It also seems evident that the rate of supply, or-the time within which 
the force acts, must have a bearing in this case, and it is imaginable that the 
upward propulsion of the magma might be so rapid that a small laccolith could 
be formed where the arch of the strata was such that it was within the limits of 
plasticity and would tend to maintain itself after the upward force ceased, 
even though the magma was in an extremely liquid state. 
Also, there is with a given source of supply and a given viscosity a certain 
limit beyond which a lava cannot form a sheet, but, if the supply of material 
continues, must form a laccolith. For at a certain radial distance from the 
supply the internal viscosity, assuming even that its ratio remains the same, 
will check the transmission of pressure and the onward-propelling, splitting 
force of the lava; but, the supply continuing, the strata must uparch and form 
a laccolith. 
Pirsson recognized other factors in the problem besides that of 
viscosity, but they have not a direct bearing here. If any contri- 
bution is made at this time, it lies in the suggestion of the function 
of progressive increase of viscosity and its effect on form and in sug- 
gesting that, though varying end results may seem to be types which 
at first sight call for separate classification, they are in fact but 
stages in a process, the underlying forces of which are generically 
alike. To be precise: If we select Crow Peak as a type which 
superficially has characteristics of a volcanic plug as hypothesized 
by Russell (from descriptions of Newton and Winchell), we reach the 
conclusion that on a laccolith there was developed a form truly 
pluglike and that faulting is present of a sort fitting well that which 
circumscribes a bysmalith. And we suggest that such a cycle of 
phenomena may be in large measure the result of progressive 
increase of viscosity during the intrusion of a laccolithic mass; and 
further, that a series with Shonkin Sag at one end, the convex 
forms of the Henry Mountains in an intermediate portion, and the 
concave forms of the Judith Mountains at the other end, may be 
due largely to the same influence. 
