182 



E. A. STEPHENSON 



To sum up then, the alteration involves the change of the feldspars to 

 sericite, calcite, and quartz; of augite to calcite and chlorite; and of biotite 

 to chlorite, sericite, and rutile. 



Chloritic alteration appears to precede sericitic alteration and 

 to require less intense or less prolonged action or solutions of a 

 different character, so that sericite is found closer to the veins than 

 chlorite, and chlorite dominates farther from the veins. It may be 

 in place here to note that many consider the solutions which ema- 

 nate from the magma in its final stages acidic in character while 

 others hold that they are alkaline. A study of volcanic gases and 

 of the sublimates present in craters indicates that the volcanic 

 vapors are quite certainly acid, and Day and Shepherd 1 found that 

 the magmatic waters which they collected from the small dome 

 within the crater at Kilauea were acid. However, to conceive of 

 these solutions as remaining acid for a long journey through rock 

 masses after their escape from the magma requires a high degree 

 of acidity, and geologists have been loath to accept such a view. 



It is clear then that the experimental formation of these altera- 

 tion products from the feldspars and ferromagnesian minerals will 

 give some clew as to the nature of the solutions and the tempera- 

 ture and pressure conditions that obtained during the deposition of 

 the associated primary ore bodies. A study of the literature bear- 

 ing on the subject of the hydro thermal alteration and syntheses of 

 various silicates leaves the reader somewhat at a loss to discern the 

 geologic application of much of the data. Some of the experi- 

 mental work is of great interest chemically but not geologically. 

 Hydrothermal investigations that begin with mixtures of oxides 

 instead of with distinct mineral species may have in many cases 

 comparatively little geologic significance. When the definite min- 

 eral can be prepared synthetically from pure materials, nearly ideal 

 conditions may be obtained, with results that are beyond question. 

 However, since many of the minerals whose investigation is of 

 paramount interest and importance in their relation to the analysis 

 of geologic processes cannot be prepared synthetically, the next 



1 A. L. Day and E. S. Shepherd, Bull. Geol. Soc. Am., XXIV, 593. 



2 This literature is ably reviewed by G. W. Morey and Paul Niggli in Jour. A in. 

 Client. Soc, XXXV, No. 9 (September, 1913), 1106-30. 



