184 SMITHSONIAN MISCELLANEOUS COLLECTIONS [VOL. 45 



and judging from the character of the material submitted for study, 

 its surface rocks are in a remarkably perfect state of preservation, 

 as compared with those collected from the mainland. Why this 

 should be so is rather difficult to say off-hand, though it seems most 

 plausible to assign it to recent glacial scouring, the island being 

 a typical nunatak. A short distance west of Kook glacier, on the 

 mainland of Nugsuaks peninsula, the Kome plant beds are described^ 

 as overlying probable older beds, lying between gneissoid hillocks. 

 These gneissoid terrains are assumed by me to be Archean and of 

 undoubted continuation with that gneiss, to be directly described, 

 which constitutes the entire southern portion of Umanak island, 

 stretching back from the coast in low-lying irregular masses for a 

 mile or so. 



UMANAK ISLAND 



Gneiss (Cat. No. 75,478). — This rock is typically gneissoid in 

 character, consisting of roughly alternating layers of quartz, asso- 

 ciated with feldspar and biotite. At times these layers may be 

 traced completely through an ordinary hand specimen. At other 

 times they pinch out, or merge into each other, forming broad 

 bands of dark and light colored constituents. These are bounded 

 by slightly narrower bands of pure quartz and feldspar. This latter 

 constituent is apparently fresh. It is light pink in color and may 

 be readily recognized by its numerous glistening faces from the 

 base and brachypinacoid. The quartz may be readily recognized 

 by its glassy appearance and lack of cleavage. The micaceous con- 

 stituent is perfectly black ; in spots, where alteration has occurred, it 

 is golden brown. Withal the specimen appears as though it might 

 have come from a zone far within the crust of the earth, instead of 

 from its surface, as is the case. 



When viewed in thin section, the structure is typically hypauto- 

 morphic granular, consisting of an irregular mosaic of the constit- 

 uents already noted with feldspar as the most abundant mineral. 

 Excepting an occasional microline, the feldspar is nearly all ortho- 

 clase and albite. It frequently exhibits perthitic intergrowths, the 

 interlamination, at times, being exceedingly fine. Slight alteration 

 has resulted in the usual products, kaolin and sericite. These 

 occupy, at times, the entire grain ; again occurring in irregular lines 

 or patches or simply scattered along cleavage lines or the bounding 

 planes of twinning lamellae. When in the last position, the included 

 particles are frequently oriented normal to the bounding planes. 



- ' Op. cit., p. 346. 



