22 Notes on the Diabase Rocks 



Assuming that the above calculation indicates the compo- 

 sition of this Diabase-porphyrite, a question remains as to the 

 constitution of the felspars. This question, in fact, is — are 

 the potassa, soda, and lime felspars associated together, or 

 do they occur singly in this rock? Keeping in view Tscher- 

 mak's theory of the felspars, there are, I think, three 

 alternatives. 



1. The potassa, soda, and lime felspars may be combined 

 as a triclinic felspar, which occurs both as porphyritic and as 

 microscopic crystals. This alternative is not, it seems to 

 me, at all probable, for if the three are combined in a 

 triclinic form, the potassa felspar would, I think, be most 

 likely to be associated with the soda felspar as a potassa- 

 bearing albite, which would then, in combining with the 

 lime felspar, be in the proportion of albite 2 '21 to anorthite 

 1*0 — in other words, an oligoclase. 



2. The potassa felspar may be in the ground-mass as 

 orthoclase. The microscopic observations, that all the 

 felspars in the ground-mass appear to be triclinic, negative 

 this alternative. 



3. The final alternative is that in the miser oscopic felspars, 

 the potassa felspar and the soda felspar form together a 

 potassa-bearing albite. For if the porphyritic plagioclase 

 is taken in accordance with the microscopic data to be 

 andesine, and of the normal constitution — namely, albite 1*0 

 to anorthite 1*0 — then there remain for the microscopic fel- 

 spars, soda felspar and potassa felspar in the proportions of 1*17 

 to l'O. A felspar of such a composition is not improbable, 

 although, judging from the analysis given by Rammelsberg, 

 Dana, and other authorities, it might be expected to be 

 monoclinic rather than triclinic. Yet the late researches of 

 Fouque* show that in the lavas of Santorin the porphyritic 

 felspars are more basic than the microscopic prisms of 

 felspar in the ground-mass, and that, moreover, the felspars 

 have crystallised in the order of basisity. In the lavas of 

 Georgios, where the porphyritic crystals were labradorite, or 

 even anorthite, the numerous minute felspars in the ground- 

 mass were albite, with perhaps a little oligoclase. It is 

 noteworthy that in the analysis given of the albite, there 

 is 1*33 per cent, of potassa evidently isomorphic in the 

 compound with soda, while in the porphyritic crystals there 

 is but *08 per cent. 



* Fouqu6. — Santorin et ses Eruptions, Paris, 1879. 



