64 H. F. Held — Studies of 3Iuir Glacier. 



of mount Cook by Mr I. C. Russell, of the United States Geo- 

 logical Survey, during the same season is also incorporated with 

 the others. 



Thanks are due to the director of the United States Geological 

 Survey for his kindness in authorizing the preparation of the 

 microscopic sections necessary for this investigation. 



No elaborate or exhaustive study of this material was under- 

 taken: first, because a preliminary examination showed that 

 neither the intrinsic interest of the rocks nor their state of preser- 

 vation warranted it ; and still more because the bare petrograph- 

 ical description of a lot of disconnected hand-specimens, with no 

 information in regard to their occurrence or geologic associa- 

 tion, is of but doubtful value. 



The real interest of this rock collection consists in the light it 

 throws on the nature of the Alaskan mountains, which are for 

 the present at least inaccessible to the geologist, and in the strong 

 similarity between these specimens and the rocks which have 

 been more carefully studied from portions of the Cordilleras and 

 Great Basin farther southward.* 



While none of the specimens in this small series give any clue 

 to the nature or horizon of any sedimentary deposits, we do firid 

 in the coarsely crystalline diorites, quartz-diorites and gabbro a 

 correspondence to the most ancient and probably Archean ter- 

 ranes occurring in the regions farther southward which are cut 

 by dike rocks very similar to if not identical with those collected 

 on the Muir glacier by Professor Reid. Although in the absence 

 of geologic data this correspondence is only an indication, 

 still it is too apparent to be overlooked. In the succeeding 

 petrographical descriptions I shall therefore distinguish between 

 the more coarsely crystalline plutonic rocks, whose structure 

 indicates their occurrence in large masses, and the finer-grained 

 though still for the most part holocrystalline rocks derived in 

 all probability from dikes. 



These latter rocks, as is usual in such series, cover a wide 

 range, both structurally and mineralogically. The difi'erent 

 types pass almost too gradually into one another to allow of 

 any sharp division; and yet for convenience of description, 

 rather than because they stand for any petrographically well- 

 defined groups, they may be ranged under quartz-porphyry 



--See J. P. Iddings : Bull. U. S. Geol. Surv., no. ()H, 1890, p. U. 



