ON THE OIB EED SAISTDSTONE OE THE NOETH OE IRELAND. 529 



36. On the Old Eed Sandstone of the Noeth of Ieeland *. By 

 Joseph Nolan, Esq., M.E.I.A., of H.M. Geological Survey of 

 Ireland. (Eead June 23, 1880.) 



(Oommunicated by Professor Hull, LL.D., F.E.S., F.G.S.) 



In the North of Ireland there are two distinct kinds of rock, classed 

 both on Griffith's and Portlock's Geological Maps as Old Eed Sand- 

 stonef. The lower and larger member of this group occupies a con- 

 siderable area, having an extent of about thirty miles in length by 

 an average width of ten miles, from Lough Erne north-eastwards to 

 Pomeroy in Tyrone. It consists for the most part of dark red and 

 purple conglomerates, often coarse and massive, and of purple, and 

 sometimes, though rarely, greenish- grey, pebbly and fine-grained 

 sandstones, often micaceous and in some cases calcareous, with sandy 

 shales. The pebbles in the conglomerate, which vary from the 

 smallest size up to blocks over a foot in diameter, consist of purple 

 felstone, grits, schist, and quartzite. Of all these the felstone pebbles 

 are by far in greatest proportion, the rock being in some places 

 almost entirely composed of them ; and their source is unquestion- 

 ably certain tracts of igneous rock which will be presently 

 described. 



On the north and north-west these conglomerates, sandstones, &c. 

 are bounded by metamorphic rocks, from which they are separated 

 by a fault ; but in the north-east, near Pomeroy, they rest uncon- 

 formably upon fossiliferous slates and grits of Lower Silurian age. 

 Though the discordance between these formations is not actually 

 seen in section, yet there can be no doubt of its existence, the 

 southern extension of the Lower Silurian rocks occupying a semi- 

 circular area, in which the strike is east and west, while the red 

 sandstones that margin it on the west, south, and east strike re- 

 spectively to the N.W., E. and W., and N.E., thus proving their 

 deposition around what appears to have been a low-lying cape or 

 projecting point of the ancient Cambro-Silurian land. 



Associated with these red sandstones and conglomerates are 

 several tracts of igneous rock, which appear to have been submarine 

 lavas, poured out at various periods during the formation of the con- 

 glomerate, the greater part of the pebbles in which, as before re- 

 marked, are evidently derived from them. They are basic felstones 

 of a purple colour and in general an earthy aspect, seldom exhibit- 

 ing any crystalline structure, but compact, with vesicles in many 

 places, and occasionally crystals of felspar and prisms of hornblende. 

 Thus it may be seen that the rock answers very closely to Cotta's 



* This paper is published with the permission of the Director-General of 

 the Geological Survey. 



t It is right to mention that in a note on Griffith's map it is stated that the 

 Zower of these divisions " may possibly belong to the Silurian system." 



Q.J.G.S. ^0.144 2o 



