WETROGRAPHICAL NOTES. $5 



Vnicroerystalline variety, to the absence of the foreign particles of quartz 

 which appear to have started the growth of the " quartz mosaic." 



A consideration of the appearances presented by the fluidal struc- 

 tures so beautifully developed in several of these rhyolites (PI. X, 

 figs. 3 — 5) seems to me to lead to the same conclusion that I have 

 deduced from a study of the " quartz mosaic," that is to say, that the 

 rocks at the time of their consolidation were not in the condition of 

 a homogeneous glass, which has become crystalline by subsequent 

 devitrification. In all the slides which exhibit this structure we find 

 alternate bands of exceedingly fine grained, and more coarsely crys- 

 talline matter. The coarse grained bands are never continuous for 

 any considerable distance, but are always lenticular, while the fine 

 grained bands stream round these lenticular areas just as they do 

 round the phenocrysts. If the coarse grained lenticles are closely 

 examined, it will be seen that the crystals composing them have 

 grown inwards from either wall, and that there is a well marked line 

 along the centre, where the crystals from either side meet ; in fact 

 the structure is exactly similar to that of an ordinary mineral vein. 

 In some cases, as in No. 1 1565 (PI. X, fig. 3), the substance filling the 

 central space of the lenticle is hornblende, and in others, it appears to 

 be either magnetite or a mixture of magnetite and hornblende. The 

 crystals projecting from the walls are also frequently hornblende, 

 sometimes in rod shaped microlites and sometimes in radiating 

 bunches of delicate needles (PI. X, figs. 3, 4). It appears therefore 

 that the magma filling these lenticles contained a larger amount of 

 ferromagnesian material than the body of the rock, and was presuma- 

 bly in a more liquid condition. This would account for the more 

 perfect growth of the crystals in these areas, just as the growth of 

 the crystals in a cavity filled with concentrated mineral solution is 

 generally more perfect than in the surrounding rock. On the other 

 hand, on the assumption that the rock was originally a homogeneous 

 glass, it would be difficult to account for the presence of these lenticles. 



( 85 ) 



