96 Cote—The Rhyolites of the County of Antrim ; with a Note on Bauxite. 
of the lithoidal type of Carnearny occurs opposite the house which stands at the 
summit of the Doagh and Connor road; and it crops out also over the broad 
eastern slope of the brae immediately to the south. 
The varieties of glassy rhyolite on Sandy Braes may be best appreciated by 
the microscopic examination of a few samples. It may be convenient to 
consider first the more distinctly perlitic types, such as have been studied so 
carefully by Mr. W. W. Watts.* One of these glasses, which I obtained in situ 
on the north side of Sandy Braes, has become grey through the perfection of its 
perlitic structure. Under the microscope (Pl. IV., fig. 3), the glass is brown, 
somewhat contrary to expectation, and has a slight tendency to become pumi- 
ceous, the minute bubbles being elongated and marking the direction of flow. 
The perlitic cracks are of course subsequent to this fluidal structure. Among the 
porphyritic crystals, by far the most abundant is quartz, rounded and corroded 
by the glass, but with some traces of its bipyramidal form. Orthoclase is 
fairly common, but plagioclase seems rare. A decomposed greenish prismatic 
mineral, resembling some “‘ bastites,” is the only ferromagnesian constituent that 
I have seen under the microscope. It is probably pyroxenic. 
The characteristic curvilinear cracks in the quartz—often like circles that 
have met but failed to intersect—have been described as perlitic by Mr. Watts, 
and he has drawn the conclusion that perlitic structure, occurring in a now 
crystalline groundmass, is no proof of the originally glassy condition of the rock. 
There is often, indeed, a resemblance between the cracked quartz-grains and the 
poorer areas of the perlitic groundmass ; but I cannot agree that the cracks in the 
quartz are ‘at least as perfect as those produced by the rapid cooling of Canada 
balsam.” The most beautiful perlitic material with which I am acquainted is the 
specimen of Canada balsam prepared by Mr. F. Chapman, and figured by him in 
the Geological Magazine. In my own slides I find cases where the film of glass 
next the quartz-grain is far more delicately perlitic than the average groundmass; 
but the cracks are entirely in the film, and do not enter the enveloped crystal. 
It has long been known that perlitic structure occurs subsequently to the 
fluidal and other primary structures in a cooling lava, and when the rock is 
practically solid; but I fancy that Mr. Watts underestimates the amount of glass 
present in lithoidal specimens which contain perlitic structure. He states, on the 
other hand, that in the lithoidal variety which he selects for descriptiont ‘it is 
very rare to find anything approaching a perlite,” ¢.e. perlitic structure. The perlitic 
cracks in this specimen occur round and also entering the porphyritic crystals, 
where differences in expansion and contraction are most marked. Mr. W. F. 
* Op. cit., Quart. Journ. Geol. Soc. London, vol. 1., p. 367. 
} ‘On a method of producing Perlitic and Pumiceous Structures in Canada Balsam,” Geol. Mag., 
1890, p. 79. { Op. ectt., p. 378. 
