148 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK. 



Now, there is ample evidence, both in the ejected lavas and in the coarsely 

 crystallized rocks in the conduit, that water vapor was uniformly and gener- 

 allv distributed throuerh the whole series of molten mag-mas, and there is 

 no evidence that there existed in the magmas which stopped within the 

 conduit any more vapors than those which existed in the magmas that 

 reached the surface, or that they were different in the two cases. Hence 

 we conclude that: 



The efficacy of these absorbed vapors as mineralizing agents teas increased 

 by the conditions attending the solidification of the magmas within the conduit. 



Moreover, if mineralizing agents are universally present in igneous 

 magmas, and if their action, so far as we can observe 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 particular eruption. 



