REPORT OF THE CHIEF ASTRONOMER • 773 



SESSIONAL PAPER No. 25a 



difficult to prove that the ' shonkinite ' shells of b and c is not really a granular 

 continuation of the porphyritic shell a. All three shells may represent the 

 original magma, which in the center has differentiated, giving shells d, e, f and 

 g. The analyses of b and c have not been published. 



c A 



Figure 42.— Illustrating two methods by which basic contact-shells in a stock (a and b) or a dike 



(c and d) might be formed. Arrows show directions of movement of salic (S) and femic (F) 



constituents during differentiation. . . 



Diagrams « and c represent the prevailing conception of contact-basification, that is 



through diffusion of more basic material to cooling surface. 



_ Diagrams b and cl illustrate the hypothesis that the basic contact-shell represents magma 

 which was chilled at the molar contact and was not so thoroughly differentiated into salic and 

 femic poles as the middle part of the mass. 



In a and b the heavy black line represents the basic contact-shell ; in b it is broken at the 

 roof, to suggest the probable effect of resurgent gases in furthering differentiation. 



Whatever be the exact method of the differentiation, the high probability 

 of gravitative control is shown by Pirsson's ably assembled facts. Further, as 

 his monograph shows, this conception gives the key to the origin of many other 

 igneous bodies in the Highwood mountains. 



25a — vol. iii — 50i 



