of the North East of Ireland, 191 



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 

 purhice stone. Radiated zeolite is the only fossil I am acquainted 

 with that resembles pcarlstone in the characters of fusion. The 

 specific gravity of two different specimens, 1 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 rhe 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, im- 

 mediately 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 re- 

 ferable to the class of transition or primitive rocks, and regard their 

 appearance in this situation as the result of a vast denudation which 



