VI. 



The Causes of Variation in the Composition 

 of Igneous Rocks. 



FEW questions occupy a more important place in the mind of the" 

 physical geologist at the present time than why, within certain 

 limits, the composition of eruptive rocks varies. Numerous opinions 

 have at different times been expressed, but none of them have 

 stood the test of time, nor have they at any epoch been universally 

 accepted. 



When we come to consider in general these different views — 

 maintained, many of them, by both careful observers and clear 

 thinkers— we find that they fall into two groups. There are those 

 who hold the existence of two extreme pastes, the mixing together 

 of which affords all the intermediate varieties of eruptive rocks ; 

 while the opposite school are equally confident that from a single 

 original paste have been derived the numerous varieties of igneous 

 rocks. 



Numerous theories have been suggested to explain the differen- 

 tiation of the two extreme pastes as held by the first school, and these 

 explanations, with others, have been based on the action of the 

 same agents as those which differentiate the common magma believed 

 in by the second school. 



Many of the hypotheses grade into each other, and some authors 

 hold more than one agency at work ; while others are somewhat 

 mixed in their ideas as to the influence of one cause as separate 

 from another. 



As for the first school, they maintain that either whole or part 

 of the fluid rock portion of our earth was, at one time or another, 

 separated into a number of concentric shells, just as we find air 

 separate from water. 



This, at first sight, seems highly plausible ; and I think we can 

 concede they are right when they consider forms of matter of totally 

 different molecular structure, such as air, water, oxides, and metals, 

 just as we could confirm the same truth with such substances as 

 mercury, chloroform, water, and oil, which remain separate when poured 

 into a tube, in the order of their density, and, however much they 

 are mixed together, separate again. 



