382 Transactions. — Geology. 



containing quartz the absence of tridymite may be explained 

 on the hypothesis that steam was not present in sufficient 

 quantity or of a sufficiently high temperature or tension to 

 dissolve the silica. 



The juxtaposition of sanidine and plagioclase can easily be 

 explained by assuming that under like conditions the mole- 

 cules of plagioclase would have greater tendency to accumulate 

 than those of sanidine, and the plagioclase would accordingly 

 tend to crystallize first. With slightly-changed conditions 

 sanidine might also crystallize, and because of the almost 

 complete isomorphism existing between it and plagioclase it 

 would crystallize round the already-formed core of plagio- 

 clase. 



Possibly there is a gradual increasing acidity in the plagio- 

 clase from centre to margin, as w^ell as a gradual change from 

 the base NaaO to KoO. This would better explain the presence 

 of the mantle of sanidine, for during crystallization there 

 would not only be a gradual increase in acidity in the suc- 

 cessive layers, but also a corresponding gradual change in 

 crystalline form. The centre of the core of plagioclase might 

 then be very basic, while the outside rim might be acid sani- 

 dine. The occasional presence of an intermediate differently- 

 oriented layer of plagioclase between the central core and the 

 sanidine lends this view considerable support. Again, the 

 boundaries of the plagioclase very seldom show that sharp 

 definition that would be expected if the change was sudden ; 

 and frequently there appears to be no actual line where the 

 plagioclase ends, but it appears to pass laterally into the 

 sanidine. 



The nomenclature of the rock presents considerable diffi- 

 culty, for it seems to form a connecting-link between three 

 classes of lava. 



If classification according to percentage of silica is adopted, 

 'the rock must be classed with the rhyolites or liparites, 

 though the presence of quartz crystals is generally adopted as 

 a specific character. 



If Eosenbusch's classification is adopted, we should hesi- 

 tate as to whether the rock should be called a trachyte or an 

 andesite, the distinction hinging mainly on the preponderance 

 of sanidine over plagioclase, or vice versa ; and as, according to 

 analysis, the latter appears to be in excess, the rock would 

 have to be classed with the andesites. 



According to its mineralogical composition, a place might 

 be assigned to it among the dacites ; but, at the same time, 

 tridymite would have to be considered a mineralogical equiva- 

 lent of quartz. The absence of any ferro-magnesian con- 

 stituent must, however, tell largely against its inclusion in this 

 class. 



