26 Scientific Proceedings, Royal Dublin Society. 
of very tine hexagonal tablets of Tridymite,* that volcanic form 
of silica first detected by Vom Rath} and then found by many 
others in different volcanic rocks. The hexagonal tablets are 
partly single, but also occur in groups of two, three and more 
individuals. Some tablets show on the border of the hexagons 
small faces of the hexagonal pyramids as represented in figure 1]. 
The tablets are generally covered with a light yellowish crust, 
due to decomposition of tridymite, and consisting of opal or 
hyalithlike matter. Very few of the tablets are clear and 
pellucid. 
Fig. 1. 
Microscopical Analysis.—The microscopical examination of the 
rock yielded Sanidine, Clinoclase, Tridymite, Quartz, Biotite, 
Magnetite, Epidote, Apatite and the microcrystalline paste. 
The latter, shows under the microscope a very undeterminate 
structure, but one can see, principally under crossed Nicols, that it 
is a very intimate mixture of minute grains of felspar, of quartz 
and of tridymite. The particles of quartz are always visible by 
the distinct chromatic polarisation; the tridymite, of which I 
shall speak afterwards, is not so easily to be detected, but it is 
found everywhere abundantly in the paste. The structure of the 
paste is what German petrologists call, with allusion to the paste 
of the true prophyries “microfelsitic,” but has become in many 
parts already really microcrystalline. I could not find in any part 
of it, distinct traces of an amorphous or glassy base, only the inter- 
spaces of the quartz and felspar are in part of glassy matter. 
The parallel disposition of the little bands and stripes of the paste 
give to it here and there a sort of fluidal structure. The sections 
of sanidine are very clear and transparent when thin. They con- 
tain many empty cavities and glassy interspaces with fixed bubbles. 
* Professor E. Hull has published the fact of my discovery of Tridymite in this 
rock in the Journal of the Royal Geological Society of Ireland, 1877, vol. xiv. part 
page 227. 
+ Poggendorff’s Annales cxxxy. 437. 
