242 Scientific Proceeding.?, Royal Dublin Society. 



east of the country ; the latter being part of the Tyrone rocks, 

 which are probably the equivalent of the Areniy, or perhaps 

 Cambrian. 



Associated with the Arenigs of Slieve Grallion are intrudes 

 of granite, elvan, porphyry and felstone ; while, in places both 

 in these and the Ordovicians, are whinstone dykes of different ages. 



The granite is not a durable rock, the exposures are of small 

 extent; and as they adjoin the sandstones, there is no necessity to use 

 them for building purposes. Some of the granites are handsome 

 stones, especially those of Carndaisy ; but Egau is of opinion that, 

 as they are so much jointed, they could not be raised in blocks of 

 sufficient size to be profitably used for ornamental purposes. 



The elvan and felstone are in the neighbourhood of the granite : 

 they, too, are not in repute. Some, however, are handsome ; but 

 as they are in out-of-the-way situations, and would be expensive to 

 work, it seems unnecessary to dwell upon them further. 



The Whinstones occur as dykes in scattered localities, and are 

 not generally used as building materials ; they are, however, 

 emj)loyed as road metal. 



Within the area of its exposure, the schist is the stone princi- 

 pally used for walling. It rises in flat-bedded masses ; and as the 

 joints are generally more or less regular, they give good faces for 

 building purposes. This rock cannot, however, be dressed across 

 the grain. 



At Prehen (vol. v., p. 581), and several other places in the 

 vicinity of Londonderry, there is a " book schist," or a rock with 

 even laminae, which give the transverse section the appearance of 

 a number of leaves of paper laid one on another in a pile. This, 

 has been extensively used in the town and its suburbs. Near 

 Limavady the schist contains a good deal of iron, which comes out 

 under the influence of the weather, giving the walls a dingy 

 appearance. AtLislane, S.E. of Limavady, the schist was quarried 

 for the railway works, (g. s. m.) Near Muff is an argillaceous 

 schist {phyttyte), which rises in coarse, heavy slates. Elsewhere in 

 the area are numerous quarries, the schists in these varying greatly, 

 being micaceous, argillaceous, hornblendic, quartzytic, &c, but all 

 more or less suitable for walling. 



In the eastern part of the county, and extending in that 

 direction into the Co. Antrim, and southward into Tyrone, are 



