IGNEOUS ROCKS IN YELLOWSTONE PARK. 
219 
portion of the flow the whole mass is pumiceous but in differ¬ 
ent degrees, and the presence of absorbed vapors may be 
detected chemically and physically in the compact layers. 
Its amount, however, was not sufficient to produce complete 
crystallization under the attendant physical conditions. 
Its effectiveness in this case was controlled by the geological 
occurrence of the magma. 
It is to be observed, in addition, that, whatever the min¬ 
eralizing vapors in acid magmas may be, there is the same 
evidence of their existence in intermediate and in basic 
magmas, whether we investigate them chemically or phy¬ 
sically, or study the phenomena of their geological occur¬ 
rence. There are even indications of their greater abund¬ 
ance in the basic lavas, many of whose glasses contain a 
high percentage of water, and the highly vesicular character 
of whose lava-flows is universal. Nor are the geological 
evidences less conclusive that demonstrate the existence of 
abundant explosive agents in the basaltic and andesitic mag¬ 
mas that have hurled their shattered masses over broad 
areas of country, and have piled vast accumulations of 
basaltic breccia throughout our western territory. 
Nevertheless, with all these evidences of the universal 
presence of mineralizing agents in basic magmas, we do not 
recognize their influence upon the microstructure or crys¬ 
tallization of basic lavas. We may assume, then, that in 
the majority of these cases they have no influence. 
But when the basic magmas become coarsely crystalline, 
and separate into minerals, the crystallization of some of 
which we have already referred to the action of mineraliz¬ 
ing vapors, we may logically assume that in these cases the 
absorbed vapors have influenced the crystallization of the 
magmas. 
If this reasoning is correct, then the action of mineralizers 
upon basic magmas is controlled by the physical conditions 
under which they solidify. 
Finally, if mineralizing agents are universally present in 
igneous magmas, and if their action, so far as we can ob- 
