214 



THE GEOLOGY OP MINNESOTA. 



[Aporliyolyte. 



Three trials with hydrofluosilicic acid give chiefly characteristic crystallites of 

 potassium by the Boricky method. Amongst these are also a few inclined crystals 

 of fluosilicate of lime, and many smaller, less brilliant, hexagonal forms, indicating 

 sodium. 



A good cleavage grain, parallel to the easy cleavage 001, shows the cleavage 010 

 by reason of the rectilinear edges of the overlapped lamellae 001; and the extinction 

 here is 2.5 to 3.5. Another grain parallel to 010 in like manner shows the cleavage 

 trace of 001; here extinction is 9.5. Specific gravity is 2.(>1. by \Vestphal balance, 

 in iodide of methylene. 



These characters agree in pointing to rn/oiilioc/<itic as the nature of the translu- 

 cent crystals in the red fluidal base of the Great Palisades. (Fouque, Bulletin de la 

 Societe de Mineralogie de France, vol. xvii, page 428.) 



Below are the forms of some of these crystals as they appear in the section 

 (figure 15): 



FIG. 15. ANORTHOCLASE IN NO. 140(7) MORE OR LESS RESORBED. 



Still, there are other glassy crystals in the same rock, situated in the red matrix 

 in a manner quite similar to the foregoing, which do not show plainly any regular 

 cleavage. They are fractured along lines quite irregular and arbitrary, and they 

 appear like quartz phenocrysts. One section is plainly uniaxial, having a hexagonal 

 outline, and must be quartz. Several others cannot be distinguished from quartz, 

 though they may be feldspar. The double refraction of quartz is so nearly that of 

 anorthoclase, and of both it is so low, that they cannot be distinguished by the colors 

 between the crossed nicols. In this rock they show clear and limpid, almost color- 

 less sections of the thickness of .02 to .03 millimeters. 



That Irving mistook these anorthoclases (which are the "adularia" referred 

 to by the Ninth Annual Report, pages 21, 33, etc.) for quartz is evident from an exam- 

 ination of his report (Monograph v, U. S. Geol. Survey, Copper-bearing Rocks of Lake 

 Superior, 1883, pages 95 to 112). He mentions, it is true, oiihodase as a porphyritic 

 constituent of this felsyte, as well as oliyrtcluse, and some little crystals are repre- 

 sented in figure 9 of plate XIII;* but in describing them he says: 



*op. at. 



