244 Scientific Proceedings, Royal Dublin Society. 



In the south of the county, and extending westwards into Co. 

 Fermanagh, are Silurians, with their associated granites, el vans, 

 eurytes, and whinstones. 



The descriptions of the Tertiary Whinstones of the Co. London- 

 derry are generally applicable to those of this county. Locally 

 they are used for building purposes, especially for quoins and such 

 like ; where they have an effective appearance, if the facings are 

 of white limestone, or sandstone. For finer work the far-famed 

 sandstones of Dungannon are more generally used. 



The eurytes and their adjuncts occur both as granites and 

 elvans in the older rocks, principally in the metamorphosed Arenig 

 or Cambrian, and as intrudes and interbedded sheets, with their 

 associated tuffs, in the Silurians [Old Red Sandstone type). 



These granites, generally speaking, although often very hand- 

 some red or variegated rocks, are shingly, or easily disintegrated, or 

 full of joints, and consequently cannot be raised in blocks suitable 

 for either general building or ornamental purposes. Some of the 

 intrudes of porphyry and euryte, however, give well-shaped, good- 

 sized blocks, and might be utilized, except that the sandstone of 

 the county, being more easily worked, is preferred by the architects 

 and stonecutters. 



A little west of JDeveney Bridge, to the south-east of Omagh, 

 there is a large exposure, for the most part a compact purplish 

 rock, which rises in large blocks and cuts, and polishes well ; it is 

 also easily plugged and dressed. In part of the mass the rock is 

 an amygdaloid, spotted white and red, a really handsome stone, 

 well worthy of being more known than it is at present. Some of 

 the eurytes also, when porphyritic, are handsome ; but they are often 

 affected by a structure that causes them to rise in thin-bedded stones. 



The bedded eurytes at Glenbeg, Shane Barnagh's Sentry 

 Box, &c, are associated with tuffs. Some of these eurytes are 

 very peculiar, as they are traversed by a structure exceedingly like 

 bedding, which subdivides them not only into slabs, but also into 

 what might even be called slates. This is very conspicuous in the 

 neighbourhood of Back Bridge, where they have been quarried for 

 slabs, for flagging, and for roofing purposes. 



There are scattered dykes of felstone in the metamorphosed 

 Ordovicians, a more considerable intrude occurring near Creggan 

 Hill, about eight or nine miles S.W. of Newtownstewart. 



