220 IDDINGS—IGNEOUS ROCKS IN YELLOWSTONE PARK. 
serve it, is controlled by the physical conditions imposed 
by the geological history of each eruption, we should not 
regard the presence or absence of certain minerals, relegated 
to the influence of mineralizing agents, as evidence of the 
presence or absence of these agents in the molten magma; 
but we should see in it the evidence of special conditions 
controlling the solidification of the magma, and should seek 
the fundamental causes of the mineralogical and structural 
variations of a rock in the geological history of its particu¬ 
lar eruption. 
