VARIATIONS OF TEXTURE IN IGNEOUS ROCKS 595 



rhyolite, to coarse siliceous granite and alaskite. The number- 

 less variations are within a few feet of one another and are 

 arranged in bands, recalling immediately the similar phenome- 

 non in the andesitic rocks of Mason Butte. In the case of the 

 rhyolitic rocks also there is no sign of intrusion of one into 

 another, nor in general does there appear to be any marked 

 gradations between the different bands at contact, the bound- 

 aries between them being fairly distinct. 



It is clear in the field that the variations are chiefly textural 

 and that the composition of all the varieties is nearly the same 

 — that of siliceous granite or rhyolite. 



In this case again, we have rocks which appear to represent 

 the roots of old volcanics, being intermediate between com- 

 pletely massive plutonic igneous rocks and superficial fine- 

 grained volcanics. They must have suffered a flowage 

 resulting in the formation of this peculiar streaky structure, 

 while the great variations in texture in the different bands show 

 crystallization at points still far removed from the surface. 



Of the different rock varieties in the locality west from Wel- 

 lington the following will be briefly described. 



Biotite-rhyolite {i^A, N^.). — Structure porphyritic; groundmass 

 cryptocrystalline, probably devitrified glass. The phenocrysts 

 are of all sizes, the larger ones grading down to those which 

 vanish in the groundmass. They are of feldspar, quartz, and 

 biotite, the latter decomposed. The feldspar was determined to 

 be largely albite and oligoclase-albite, although there is some 

 orthoclase, The largest phenocryst of feldspar measures 

 2^™"" in diameter; the largest one of the quartz, i ^^ °"°. The 

 rock contains angular fragments of finer grain. Some of these 

 seem to be devitrified rhyolitic glass, while others are fragments 

 of more basic lava, probably andesite — these latter show small 

 lath-shaped feldspar phenocrysts in a glassy semi-devitrified 

 groundmass. 



Biotite- rhyolite (165 N'^.). — In this specimen the groundmass 

 becomes slightly coarser than in the preceding specimen and is 

 very fine microgranular. It also becomes more scant than in 



