PRTROGRAPHICAL NOTES. 8 1 



phenocryst in among the minute crystals of felspar forming the 

 remainder of the groundmass, giving rise to a minutely micropeg- 

 matitic structure as in fig. 6. This structure appears to me to 

 throw some light on the question of the origin of the gioundmass of 

 rhyolites and quartz porphyries and will be referred to later on in the 

 discussion of the groundmass. 



Inclusions of various kinds are tolerably numerous in the quartz 

 phenocrysts. They include gas pores, which are often arranged 

 in lines parallel to the cracks traversing the crystals ; and glass 

 inclusion?, usually with a fixed bubble. These latter sometimes 

 possess hexagonal outlines, but are more usually rectangular with 

 rounded corners. Occasionally a faint asterism is perceptible in 

 them. Cavities containing liquid are also not rare, and these generally 

 contain a movable bubble, which when the cavity is of sufficiently 

 small dimensions, and the bubble is also small, may be seen under a 

 high power to be in constant spontaneous motion. The inclusion of 

 portions of the groundmass in the quartz phenocrysts has already been 

 alluded to. 



The felspar phenocrysts do not as a rule show signs of corrosion, 

 but frequently have well defined crystalline outlines; this suggests that 

 the felspars may be phenocrysts in the true sense of the word, that is, 

 that they have been formed in situ in the molten magma, while the 

 quartz phenocrysts may have been floated in from some deep-seated 

 coarsely crystalline mass, pierced by the rhyolites, and undergone 

 corrosion in the process. In the phenocryst shown in fig. 2, PI. X, 

 the felspar possesses perfect crystalline outlines, while the quartz is 

 apparently merely the skeleton of a previously formed crystal. I have 

 never found any trace of secondary growth at their edges, such as is 

 so often exhibited by the quartz grains. Sometimes the felspars possess 

 well defined simple twinning or consist of aggregates of interpene- 

 trating individuals, a structure that is very distinctly brought out in 

 polarised light (PI. X, fig. i). The great majority of them are un- 

 doubtedly orthoclase, and many have the " glassy " appearance of 



( 8. ) 



