198 E. A. STEPHENSON 



determinations of isotropic minerals as glass may be incorrect, and 

 that analcite has been overlooked. In examining the slides in the 

 University of Chicago collection a slide from a trachyte of Bannber- 

 scheid, Westerwald, Nassau, was found which showed the soda 

 feldspars altered to analcite though the original crystal boundaries 

 remained sharp. The alteration does not follow cleavage cracks 

 but appears in irregular patches. Mr. K. F. Mather in a forth- 

 coming paper will describe an eruptive cone of Quaternary age in 

 the canyon of the Mancos River ten miles southwest of Mancos, 

 Colorado — locally known as the " blowout " — which is cut by dikes 

 of augite minette. These dikes carry fragments of granite xenoliths 

 which are deeply corroded and partially assimilated. The feldspars 

 are altered to analcite — identified microchemically. A careful 

 study of rock specimens would probably show that this type of 

 alteration is much more common than has been supposed. 



SUMMARY 



i. Alkaline solutions of different characters dissolve the feld- 

 spars with separation of silica and crystallization of compounds less 

 rich in silica. The solutions are probably hydrolyzed since the 

 reactions are accelerated by the presence of alkalies, by increased 

 concentration of the alkalies, and by higher temperatures. 



2. Feldspars and hornblende are not appreciably attacked by 

 pure water at temperatures up to about 300 C. adularia to at least 

 350 C, showing that the dissolved substances rather than water 

 alone must cause the differences in the nature of the alterations. 



3. Albite and orthoclase feldspar seem to respond to the action 

 of the alkalies in nearly identical ways, and hence the conclusion 

 is patent that they have very similar chemical structures. 



4. The influence of small amounts of fluoride and borates as 

 mineralyzers has not been found important, at least in the presence 

 of the other substances. This leads to the suggestion that possibly 

 the mineralyzing effect is merely that of causing solution at tem- 

 peratures where the silicates in question would otherwise be much 

 less soluble. 



5. It is notable that no kaolin or kaolin-like substance forms 

 from alkaline solutions at temperatures up to 280 C. The sugges- 



