nWIJAN PETRQLOGICAL SKETCHES 551 



of tlcfinitc crystal forms, but rather the orientation of the 

 groundmass into micropoikilitic patches, though the principle 

 involved is the same in both. Similar mantles of alkali feldspar 

 about labradorite have been observed by Iddings^ in rocks of 

 the W'llowstonc Park, by Pirsson^ in the S3'enite of Yogo Peak, 

 Montana, bv Merrill^ in rocks from Montana, and by Kolenko'* 

 in trachyte from New Zealand. 



The l^ordcrs in question arc closely analogous to the micro- 

 ])oikilitic patches of quartz described by Miss Bascom,^ Iddings^ 

 and Clements, 7 especially the last. Miss Bascom shows reasons 

 for thinking the structure in the South Mountain rocks to be of 

 secondary origin, while at the Electric Peak they are primary, as 

 they also probably are in the Michigamme District according to 

 Clements. 



In the case before us the evidence is entirely in favor of their 

 |)rimary origin. As will be seen the phenomenon was observed 

 in many other rocks of the Italian volcanic regions. These rocks 

 are all comparatively recent lava streams, fresh and unaltered, so 

 that there can be no appeal to metamorphic processes. The 

 whole appearance of the border, with its included grains and its 

 identity with the groundmass (except in its individual orienta- 

 tion) , leaves no doubt that the effect is due simply to the 

 orientating influence of the feldspar phenocrysts during solidifi- 

 cation of the groundmass magma. ^ 



The remaining phenocrysts are of augite and biotite, but 

 offer few features worthy of note. In the normal gray vulsinite 

 the augites — both phenocrysts and groundmass crystals — are of 

 a pale olive green, while in the eutaxitic brown streaks they are of 

 a bright golden-yellow color. The biotite phenocrysts are pale 



'Iddings, Journal of Geology, III, 940, 941, 1895. 



^ PiRSSON, Am. J. Sci., L, 471, 1895. 



3 Merrill, Proc. U. S. Nat. Mus., XVII, 645, 1894. 



^KOLENKO, Neu. Jahib., 1885, I, 9. 



5F. Bascom, this Journal, I, 816, 1893. 



^-J.P. Iddings, Electric Peak, 12th Rep. U. S. G. S., 589, 1892. 



^ J. M. Clements, this Journal, III, 814, 1895. 



^ Both Klein and Bucca consider them primary but late growths of orthoclase. 



