Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/199

 and rounded. Their colour is smoke-grey or bluish, with a pearly lustre. They seem formed of concentric and very thin coats. The fracture of this mineral is imperfectly conchoidal; it cuts glass but faintly, and emits a faint argillaceous smell when breathed upon. Fragments, exposed to the blowpipe, intumesce to four or five times their first volume, fusing into a foamy and light glass, not unlike pumice stone. Radiated zeolite is the only fossil I am acquainted with that resembles pearlstone in the characters of fusion. The specific gravity of two different specimens, I found 2,38.

About 76 miles to the north of this district, at Ballycloghan, two miles north-west from the village of Broughshane, there is a bed of clay porphyry extending towards Slieve Mish on the south-east; it is quarried as a freestone, and when raised in thick slabs, is used for window seats.

The basis is compact and sometimes earthy, of a greyish white colour; it contains imbedded concretions of smoky quartz, lamellar crystals of white felspar, and a few interspersed plates of brown mica; it adheres to the tongue slightly, and fuses into a transparent but frothy enamel; the specific gravity is 2,43.

The occurrence of a porphyritic district, surrounded on all sides by a vast area of basalt, must be considered as one of the most singular facts which the country we have examined presents. The question to what formation do these porphyritic rocks belong, immediately suggests itself, but the materials which observation has hithereo afforded cannot be considered as authorizing any decided answer. Many geologists, among whom it will be sufficient to mention Dr. Macdonnel and Dr. Richardson, consider them as referable to the class of transition or primitive rocks, and regard their appearance in this situation as the result of a vast denudation which