ERUPTIVE ROCKS. 



313 



The Cottonwood granite, which, on the supposition advanced by Pro- 

 fessor Geikie, must have taken up an enormous amount of siUca, lime, and 

 magnesia, shows, however, no abnormal amount of these constituents in its 

 composition, which is that of a normal granite rather rich in plagioclase. ^ 



' The compositiou of this granite, as given in Exploration of the Fortieth Parallel, Vol. II, p. 

 357, is as follows: 



