37() PROF. J. AV. JUDD ON THE PROPYLITES 



Now the separation that has taken place within the Eskdalemiiir 

 dyke, on a small scale, has evidently gone on on a grand scale beneath 

 Ben Hiant. The result of this has been that, while in some of the 

 lava-sheets of the mountain Ave find a glassy lava through which 

 porphyritic constituents are somewhat sparingly distributed (the 

 result being a " pitchstone-porphyry " similar to that of the Sgiirr of 

 Eigg), in other cases masses of crystals A\dth only a comparatively 

 small matrix of glass have been poured out, giving rise to a rock of 

 far more basic character than the pitchstone varieties. 



In studying the Krakatoa lavas I was led to point out the 

 remarkable differences in the composition and appearance of rocks 

 resulting from variations in the proportion of acid ground-mass to 

 basic phenocrysts in a rock. My examples in illustration of this 

 principle were taken from Santorin, the Cheviot Hills, and Krakatoa*. 

 .But here in Ben Hiant we find, among the ejections of the same 

 vent, the most wonderful illustrations of the same principle, one that 

 has been too much overlooked in our petrographical studies. 



I shall show that among the lavas of Ben Hiant we have 

 varieties that are distinctly basic in composition, with a specific 

 gravity of over 3 and a silica- percentage of a little aboA^e 50. But 

 among the same rocks are others with a distinctly acid character, 

 having a density of only 2-45 and a silica-percentage of over 65. 

 Yet the minerals in all these rocks are identical ; the same fel- 

 spars, the same pyroxenes, magnetite, and a similar glass, are 

 found in all ; it is the variation in the relative pj^ojoortion of these 

 several mineralogical constituents which gives rise to the very 

 wide diversity alike in the aspect and in the ultimate chemical 

 composition of these rocks. 



Mineralogically these rocks exhibit, as I have said, a remarkably 

 uniform character. They consist of: — 



1 . Felspar, which is almost always either anorthite or labradorite, 

 or some form intermediate between these species. This is proved 

 by the extinctions Avhich they give in the several zones, and is 

 confirmed when Ave examine specimens of them, isolated by the use 

 of heavy liquids, for their specific gravity, and the flame-reactions 

 which they give by Szabo's method. The felspar-crystals are often 

 zoned, the different zones giving CA'idence of being of different com- 

 position, the more basic being in the centre. Inclusions are often 

 arranged parallel to these zones, and the first traces of schilleri- 

 zation are sometimes exhibited. Although the twin striation is 

 often very marked, cases of simple Carlsbad-twinning are not un- 

 common. 



2. Pyroxene, an augite, sometimes of a green colour, at other times 

 brown. There is almost certain CAddcnce that the broAvn augite is 

 an altered form of the green ; indeed everj^ gradation from the one 

 kind to the other can be found, and crystals occur Avhich arc in one 

 part green and one part brown. The original colour is certainly 



* ' The Eruption of Krakatoa and subsequent Phenomena ' (1888), pp. 30-35 ; 

 «nd GeoL Mag. dec. iii. vol. v. pp. 1-11. 



