Cote—The Rhyolites of the County of Antrim, with a Note on Bauxite. 89 
In this prevalence of biotite, the majority of the Antrim rhyolites ally themselves 
to those of Selmeczbinya in Hungary, and not to the Icelandic examples, which 
contain pyroxene.* The few examples of porphyritic plagioclase that I have 
been able to study include among them only one which yields symmetrical 
extinctions on either side of the composition-plane of the twin-lamelle. Von 
Lasaulx obtained an angle of 54°; and this example gives 43°. Mr. Wattst is 
probably justified by wider observations in recording the occurrence of albite. It 
seemed desirable to inquire further into the nature of the plagioclase, and to 
complete the study of this interesting rock, by a dissection of the mass after the 
manner of Cordier. The microscope had already shown how singly and repeatedly 
twinned felspars are present in about equal proportions; at any rate, if some 
twenty different crystals are broken up from various parts and various specimens 
of the rock, a large number must be plagioclastic. Moreover, two cases were 
found in which the minute twin-lamellz were visible on the surface of the crystals 
with a lens. These, and other fragments obtained by direct selection or by 
sifting, were thrown into a diffusion-column of methylene iodide. One solitary 
flake, which may, after all, have been a chip of quartz selected in error, sank to 
the level of 2:68; if this were a true felspar, it indicates the presence of andesine 
or even labradorite. But all the other cleavage-flakes of felspar that were 
examined, including the two actually determined as plagioclastic, swam at from 
2°57 to 2:58, there being a fair division into two series having these specific 
eravities respectively. The conclusion is that the monoclinic and triclinic felspars 
present are very closely allied species, and that we are on the critical line where 
any increase in the albite-molecule transforms soda-orthoclase into the anorthoclase 
of Férstner. Carlsbad twinning being a possible structure in anorthoclase, some of 
the so-called ‘‘ sanidines” of Tardree may even prove, on further examination, to 
be triclinic. We may safely regard the bulk of the repeatedly twinned felspars as 
anorthoclase, with here and there an albite, which may be revealed by the 
observation of symmetrical extinctions. 
The lithoidal groundmass of the Tardree rock has a specific gravity of 2°566 ; 
so that the specific gravity of the rock-mass, already recorded as 2°45 to 2°46, must 
be attributed to the abundance of drusy cavities, lined with tridymite, which are 
too small to be eliminated. 
Where drusy cavities of fair size occur in the rock, tridymite may easily be 
recognised, often coated with a brownish or a greenish crust. The anisotropic 
character of the hexagonal plates can be seen under the microscope, the colours 
being low greys, like those of leucite. On the surface of each crystal are a 
number of minute discoidal bodies, slightly yellowish, which also cluster along its 
* Backstrom, op. cit., Geol. Foren. i Stock. Forhandl., Bd. xiii., p. 637. 
} M‘Henry and Watts, op. cit., p. 80. 
