530 J. NOLAN ON THE OLD RED SANDSTONE 



definition of Porphyrite — a name which, however objectionable, it 

 may be well to employ, as serving to identify this rock with others 

 that may occur under similar circumstances. 



In some places the tops of these felstones are tuffoid, though no 

 well-marked beds or large deposits of such ejecta occur, this being 

 probably due to the subaqueous conditions under which the mass 

 was ejected, so that fragmentary accompaniments would be drifted 

 away by tides and currents, often forming deposits in other places, 

 as mentioned by Scrope in his work on Yolcanos, p. 247. 



Besides these basic felstones there are other varieties of igneous 

 rocks associated with the sandstones. These are generally 

 melaphyres, often extremely vesicular, the cavities being in time 

 filled with white and pink calcite, giving the rock a peculiar spotted 

 appearance, which few persons who have traversed the district east 

 of Omagh, where it is extensively used for road metal, can fail to 

 have noticed. The principal source of these is at Hecarson, some 

 three miles east of Omagh. Here every change in texture and colour 

 is visible ; at one part the rock is blue and compact, at another 

 vesicular, with calcite, while at the principal quarry it is mostly of 

 a purple colour, and crystalline with glistening facets of a rich 

 bronze-coloured mineral that seems to be augite. As its relations 

 to the surrounding rocks are obscure, it could not be determined 

 whether it was contemporaneous or intrusive ; the vesicular condi- 

 tion of much of the rock might lead us to infer the former ; yet if 

 so, it is strange that no fragments of it occur in the surrounding 

 sedimentary rocks, as is the case in those overlying the contempora- 

 neous felstones. 



Besides these trappean rocks there are masses of granite asso- 

 ciated with the sandstones. These, as I have already shown else- 

 where*, are intrusive through the lowest beds of this series, 

 vitrifying the sandstones in contact and converting them into 

 quartzites. It was also shown that the intrusion took place prior 

 to the deposition of the upper red quartzose conglomerate (Lower 

 Carboniferous, so-called), as in some places the basal beds of that 

 formation are almost altogether composed of its debris, a circum- 

 stance which, taken in connexion with others presently to be men- 

 tioned, is a proof of the great length of time which must have 

 elapsed between these formations. 



lied Quartzose Conglomerates and Sandstones (Lower Carboniferous 

 so-called). — These consist of conglomerates, distinguished by the pre- 

 valence of white and pink quartz pebbles in a reddish-brown base, 

 purple, reddish-brown, and grey sandstones generally micaceous, 

 and sandy shales, passing upwards into yellow and white pebbly 

 sandstones and grits. The basal beds are often very coarse, and in 

 many places so much resemble those of the lower system, that the 

 discrimination between them is often a matter of considerable difii- 

 culty. Although in some sections these rocks rest with apparent 



* See G. S, Memoir to accompany Sheet 34, and a paper " On the Meta- 

 morphic and Intrusive Rocks of Tyrone," Geol. Mag. vol. vi. no. 178. 



