SUPPLEMENT II. 

 NOTES OX SOME ERUPTIVE ROCKS FROM ALASKA. 



BY 



GEO. H. WILLIAM;^, PH. I)., OF JOHNS HOPKINS UNIVERSITY. 



CONTENTS. 



Page 



Introduction 63 



Coarse-grained Plutonic Rocks 65 



Diorite 65 



Augite-diorite 65 



Augite-mica-diorite 66 



Sanssurite-diorite 67 



Quartz-diorite 67 



Gabbro of Mount Cook (Troctolite) 68 



Fine-grained Dike or Surface Rocks 68 



Porpbyry 60 



Micropegmatite 6!» 



Quartz-porpbyr J' 70 



Porijbyrite 70 



Hornblende-poii)hyrite 70 



Augite-porpbyrite (labradorite-porphja'ite ?) 71 



Diabase 72 



Introduction'. 



The rocks described in the following notes are for the most 

 part comprised in a collection of fifty erratic bowlders and peb- 

 liles gathered near the foot of the Muir glacier 1)}^ Professor H. 

 F. Reid during his visit there in the summer of 1890. Forty-one 

 oi these specimens were examined microscopically, the others 

 l)eing too much altered to rej^ay such a study. Microscopic sec- 

 tions of twelve specimens of eruptive rocks, collected in situ near 

 the glacier by Mr H. P. Gushing, who accomjianied Professor 

 Reid, were also examined. The description of a single sj^ecimen 

 (olivine-gabbro or forellenstein) obtained from the southern side 



(63)- 



