350 PEOP. J. W. JUDD ON THE PROrYLlTES 



I. Analysis of mucli altered pyroxene-propylite from Beinn More, 

 MuU. Made by G. H. Perry in the Chemical Laboratory, Normal 

 School of Science. 



II. Analysis of augite-andesite, Gyetva, Hungary, by Wymietal 

 in Tschermak, Min. Mittheil. 1868. 



III. Analysis of pyroxene-andesite (containing hornblende) from 

 the north-east of American Flat, Washoe. Made by W. G. Mixter, 

 1875. 



While, however, it is convenient to make a broad general distinction 

 between the usually pale-coloured and lighter amphibolic varieties 

 and the dark-coloured and heavier pyroxenic forms of these rocks, 

 it must be confessed that the division into acid and basic types of 

 propylite is of no great value. Even when there is no free quartz 

 present, either of primary or secondary origin, the variations in the 

 proportion of porphyritic crystals to base lead to wide variations in 

 the ultimate chemical composition of the different rocks. In this 

 way we often find that pyroxenic rocks are of more acid character 

 than amphibolic ones. The extreme modification, too, which many 

 of these lavas have undergone leads to most remarkable changes iu 

 their colour, specific gravity, and chemical composition, and still 

 further leads to breaking down the distinction between the two types 

 which, for convenience of description, we have sought to institute. 



IV. Microscopical ChaPvActers. 



The microscopical characters presented by these lavas may be 

 summarized as follows : — 



Although many of the rocks must have originally contained much 

 vitreous or uncrystallized material in their ground-mass, yet in 

 almost every instance this glassy substance has disappeared through 

 secondary devitrification. In the majority of cases, the development 

 of secondary minerals iu the substance of the ground-mass has 

 completely obliterated the original micro-structure of the rock ; but 

 in some instances we find traces of spherulitic, fluidal, and perlitic 

 structures ; while in others the structures known as "granophyric" 

 can be detected — such as the micro-pegmatitic, the centric, and the 

 pseudo-spherulitic. 



As a general rule, it may be said that the ground-mass is the 

 most highly altered portion of these rocks, this being doubtless due 

 to the fact that the glassy matrix is less stable than the crystallized 

 constituents of a rock. The matrix frequently acquires a green 

 colour from the development of minerals of the chlorite group, and 

 is sometimes studded with crystals of the metallic sulphides. 



The porphyritic crystals of felspar, though so greatly altered, 

 can usually be found to show, here and there, traces of the pla- 

 gioclastic twinning. They are never, however, in the vitreous 

 condition of the felspars of ordinary andesitic rocks (microtine), 

 but exhibit the opacity and dull lustre characteristic of the diorites 

 and other deep-seated rocks. It was this condition of the porphy- 

 ritic felspars, with the state of the ground-mass and the features 



