ERUPTIVE ROCKS. 



313 



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

 fessor Geikie, must have taken up an enormous amount of silica, 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. * 



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

 357, is as follows : 



SiOa 71.78 



A1 3 O 3 14.75 



FeO 1.94 



MnO . . 0. 09 



CaO ............ 



MgO ........... 



K 2 O 

 Ignition 



2.36 

 . 0.71 

 . 3.12 



. 4.89 

 . 0. 52 



100. 16 



