250 



IRVING. 



ondary minerals, the alteration having proceeded from within 

 outwards, filling the interior with a maze of highly polarizing 

 zeolites or interlocking crystals of secondary quartz. The bor- 

 ders of the crystals are, however, as a rule left intact, showing 

 merely a slight kaolinization. Where the phenocrysts are in a 

 perfectly fresh condition, many of them are seen to be microcline, 

 a core of which usually remains, even in the more decomposed 

 examples. The microclines show a marked contrast to the sani- 

 dines in that the decomposition seems to have proceeded from 

 without inwards. In some cases a kaolinized rim of sanidine en- 

 closes an intermediate zone of alteration products, within which 

 again may be seen a kernel of microcline (Fig. 12). The 



Fig. 12. Phenocryst of leldspar irom quartz-asgirite-porphyry showing kernel of 

 microcline, rim of kaolinized feldspar and intermediate zone of alteration products. 



larger feldspars show inclusions of perfectly bounded crys- 

 tals of JEgirine-augite. In no instance, however, are the feld- 

 spars penetrated by the fine segirine needles of the groundmass. 

 The feldspars of the groundmass, when, undecomposed, are 

 with difficulty to be distinguished from the quartz, but where they 

 •are of considerable size, or slightly kaolinized, they show an 



