68 //. F. Reid — Studies of Muir Glacier. 



Gahhro of Mount Cnoh (Trociolite). 



In the collection of Alaskan rocks intrusted to me by Professor 

 Reid and Mr Cushing there are no representatives of gabbro, but 

 a single specimen of this type collected in the summer of 1890 by 

 Mr I. C. Russell on the southern side of mount Cook has been 

 sent me by Mr J. >S. Diller, of the United States Geological Sur- 

 vey, and may ap2)ropriately be noticed in this place. 



This rock bears the closest macroscopic resemblance to the 

 well-known forellenstein of Neurode in Silesia,* nor is the like- 

 ness less striking when the two rocks are compared under the 

 microscope. The thin sections of the Alaskan rock which I have 

 examined show an evenly granular aggregate of serpentine grains 

 and a basic feldspar, which appears from its optical properties 

 to belong to the labrador-anorthite series. The serpentine now 

 contains no trace of the original olivine from which it has evi- 

 dently been derived. The feldspar is striped with broad twin- 

 ning lamella?, and shows evidence of considerable alteration, 

 although none of the constituents of this rock exhibit any indi- 

 cations of having been subjected to any particular dynamic action. 

 Around each serpentine grain is a border of compact greenish 

 hornblende, which for considerable distances belongs to single 

 individuals. 



To designate this peculiar modification of olivine-gabbro from 

 which pyroxene is nearly or quite absent, the English petrog- 

 raphers employ a translation of the German term " forellenstein " 

 (trout-stone) troctolite. As early as 1872 Professor Edward S. 

 Dana proi30sed the name " ossipyte " to designate a rock from 

 New Hampshire of the same mineral composition. f 



Fine-grained Dike or Surface Rocks. 



A goodly proportion of the specimens examined are jine- 

 grained porphyritic rocks, covering a considerable range of types. 

 Their structure indicates that they belong to small masses, which 

 in all probability break through the crystalline complex of more 

 coarsely granular rocks above described in the form of dikes, or 

 perhaps in some instances cover them as surface flows. These 



*Vom Rath: Pogg. Ann., vol. 95,1855, p. 551 ; and A.Streng: Nenes 

 Jahrbuch fiir Min., 1864, p. 257. 



t Am. Jonrn. Sci., 3d sen, vol. iii, 1872, p. 49. 



