88 Cote—The Rhyolites of the County of Antrim ; with a Note on Bauatte. 
Mr. Player’s analysis of the rhyolite of Tardree is as follows :— 
Silica : : . (6:4 
Alumina 2 ; 5 dlelex 
Ferric oxide ; 4 Wnadis6 
Lime . ; ; , 6 
Potash . \ : a | aly 
Soda . ; 3 ils 
Water . : : Silly se 
100°3 
Hardman found *295 per cent. of magnesia and a trace of phosphoric acid. 
In addition to the previous petrographic descriptions of the central rhyolite of 
Tardree, I may state that sections show a multitude of ‘‘ globulites” and ‘‘cumu- 
lites,” associated with the delicate colourless rods that form so much of the 
groundmass. There can be no doubt that the rock, like that of Templepatrick 
churchyard, is on the verge of becoming crystalline throughout; but the above 
features point equally clearly to the existence of a residue of glass. A micro- 
granular structure appears between crossed nicols, which may be due merely to 
the distribution of minute crystals, hidden in the glass; but it suggests that much 
of the glass has broken up in the end into crystalline grains after the primary 
separation of the crystallites from it. Such a structure appears in some of the 
spherulitic rocks of Arran,* and in those of older date which have become 
devitrified, as at Wrockwardine in Shropshire ; in such cases the primary structures 
are not obliterated by this final change. A specimen which I collected N.W. of 
Skleno, near Selmeczbénya in Hungary, is spherulitie throughout, and closely 
resembles the rock of the Corriegills shore in Arran; but extinction takes place 
in conformity with the spherulitic rays, and there is no trace of granulation. 
Perhaps the latter is always a secondary structure, but may be induced at no long 
interval after the consolidation of the lava, should it again come under conditions 
of high temperature, such as are known to be favourable to devitrification. 
In one of my sections from the Tardree mass, two disrupted and almost opaque 
bodies appear (PI. IV., fig. 1), which can be little else than injured spherulites. 
These may have been formed in the first stages of cooling, and may have been 
broken up, and in part redissolved, during a subsequent period of flow. 
Idiomorphic biotite is the usual ferromagnesian constituent ; but soda-pyroxene 
might reasonably be expected in a rock of this class. It occurs, as we shall 
presently see, in a glassy lava on Carnearny, and in some of those on Sandy Braes. 
*The rock from the shore at Corriegills, for instance, described by Bonney, ‘‘On certain rock- 
structures, as illustrated by Pitchstones in Arran,” Geol. Mag., 1877, p. 506; and by Teall, ‘British 
Petrography,” p. 346, and pl. xxxix., fig. 1. 
