674 THE JOURNAL OF GEOLOGY. 



Si0 2 



A1 2 3 - 



Fe 2 3 



FeO - 



MgO 



CaO - - - 



Na 2 



K 2 - 



H 2 



98.63 99.33 99.33 98.88 



I. Greatly altered granite, Old Sterling mine. 

 II. Prochlorite, St. Christophe. 2 



III. Delessite, Zwickau. 3 



IV. Alteration product of doubtful origin (probably derived from granite), 

 Caledonia mine. 



It has been shown that the original rock was an acid granite, 

 consisting almost wholly of orthoclase, microcline and quartz, 

 with no ferromagnesian constituents. From this there can be no 

 doubt that it contained not less than 70 per cent, of silica, with a 

 large content of alumina and alkalies, and little iron, magnesia, 

 and lime. The analysis shows that the process of alteration con- 

 sisted of a decided decrease in the percentage of silica and 

 alkalies, with an equally marked increase of iron and magnesia, 

 and the addition of much water. Moreover, this has not been a 

 mere removal of some constituents, leaving relatively increased 

 proportions of the others, but, on the contrary, there has been an 

 actual addition of material from a foreign source. It is hardly 

 necessary to say that the composition of this alteration product 

 is totally unlike that of the product that would result from the 

 alteration of such a granite under ordinary conditions. Of 

 course the analysis represents, as already stated, the extreme 

 result of alteration, but it is not probable that the results would 



r The iron in I. and IV. is all calculated as ferrous, but there can be no doubt 

 that some of it is in the ferric condition. 



2 Dana's System of Mineralogy, p. 654. 



3 Ibid., p. 660. 



