Composition of Igneous Rocks. 415 



Si0 2 , and we have a section from near the base of the reservoir, 

 we would expect the border to possess the same composition as 

 the original magma and the center to be basic. Similarly all 

 sections from the middle horizons of reservoirs should show 

 little or no differentiation. This is illustrated by the accom- 

 panying diagrams. Suppose the reservoir, for the sake of easy 

 representation, to be cylindrical. The four upper figures rep- 

 resent sections through a differentiated massive resulting from 

 a magma whose original composition was acid — the first figure 

 representing a vertical section ; the other three horizontal 

 figures form upper middle and lower horizons. Similarly the 

 four figures in the second row represent sections of a massive 

 whose original magma was of intermediate composition. The 

 third row represents sections derived from a stock whose origi- 

 nal composition was basic. 



A priori we should expect that the greater number of 

 exposed sections of eruptive stocks would be from the middle 

 horizons, that a much smaller number would represent the 

 uppermost horizons and that a very small number would repre- 

 sent the lowest horizons. This agrees exactly with the facts as 

 revealed by geological field work, viz., most stocks as exposed 

 show little or no differentiation, a much smaller number have 

 basic borders and acid centers, while a very small number show 

 acid borders and basic centers. 



The weakness of this theory consists in its fundamental 

 principle — that gravitation causes a concentration in some solu- 

 tions. Some physicists deny this, but the men who have inves- 

 tigated it most are quite confident that differentiation does take 

 place. 



Indian Museum, Calcutta, Nov. 10th, 1897. 



