GEOLOGY AND PETROGRAPHY OF THE PROSPECT INTRUSION. 489 



These features are those characteristic of secondary micro- 

 perthite, the albite having in course of time separated from 

 a soda orthoclase. That this interpretation is correct in 

 this case is confirmed by the fact that the patches showing 

 albite twinning are often bounded by cleavage cracks, 

 which are well marked as the section is thin. The only 

 other interpretation would be that the orthoclase had 

 turned in patches into microcline not albite, perhaps during 

 the process of grinding the section, but the fact that there 

 is no trace of a cross twinning is against this view. Both 

 the secondary twinning and the R.I. of the untwinned 

 felspar favour the conclusion that the mineral was originally 

 an orthoclase rich in soda. A careful search has failed to 

 reveal any primary microperthite, appearances which at 

 first sight resembled it being always more satisfactorily 

 explained on closer examination as of secondary origin. 

 The sharp separation into alternate sinuous layers which 

 characterises primary microperthite is nowhere to be seen 

 even with a high power. At the same time this does not 

 preclude the possibility that the originally apparently 

 homogeneous soda orthoclase is a cryptoperthite. 1 The 

 alkaline outermost zone of the plagioclase crystals is 

 identical in every property with the soda orthoclase 

 individuals, and is almost certainly of the same composition* 



The decomposition products of the two felspars are 

 different, and render them easily distinguishable in ordinary 

 light. The alkali felspar decomposes chiefly to kaolin, and 

 thus has the brownish cloudy appearance so familiar in the 

 granites. It has also decomposed to analcite, sometimes 

 here and there in patches, but often completely, so that a 

 large patch of analcite may be recognised by the dusty 

 kaolin and cleavage cracks to be a complete pseudomorph 

 of a felspar crystal. The plagioclase crystals, on the other 



■ Rosenbusch, Mikr. Phys. 3rd edition (1892) Vol. i, p. 678. 



