Ivinahan — On Irish Arenaceous Rocks. 605 



measures and Calp Sandstone are brought together by a fault, a 

 downthrow to the south-eastward. 



[All these lithologically similar rocks to the north-east of the Blackwater (Aughna- 

 cloy) are called on the new maps Calp, while west and south-west of that river they 

 are called by the unappropriate English sub-group names, Yoredale beds and Millstone 

 grits.'] 



The well-known sandstones of the Co. Tyrone are all of Car- 

 boniferous age ; but they may belong to the Lower Carboniferous 

 Sandstone, the Calp, or the Coal-measures. The rocks in the 

 neighbourhood of Aughnacloy, as already mentioned, may belong 

 to either of the latter groups; here, provisionally, they will be 

 described with those of the Calp. The Calp is of the two types, 

 the ordinary, and the "Ulster type;" the rocks in these will be 

 given separately. 



[The subdivisions of " Upper and Lower Calciferous Series" adopted in the Geolo- 

 gical Survey Memoirs are only lithological ; the reddish pebbly rocks forming the latter. 

 These dark-coloured rocks may, however, occur on any geological horizon, their colour 

 and composition being solely due to islands, or other shore lines in the Carboniferous 

 sea, they always beiDg found adjoining a protrude of the older rocks.] 



These rocks have been used in the county — very generally in 

 Dungannon, Coalisland, Clogher, Omagh, Cookstown, Castlederg, 

 and Caledon; while in Strabane, and other places in the schist 

 regions, they are used for quoins and other dressed-stone purposes. 

 At Baronscourt they were used, except the Portland stone for the 

 staircases, and in a few other places. Out of the county they have 

 been extensively used for cut-stone purposes. 



Near Benburb, at the south margin of the county, are sand- 

 stones that have been said to be of Permian age ; but on account 

 of the assemblage of fossils in these and the associated rocks, and 

 also of their position, Baily and the writer have suggested that they 

 must belong to the Carboniferous. 



In the northern portion of the county, at Cookstown and Kil- 

 dress, at Omagh and south-east of Strabane, are tracts of Calp, of 

 the "Ulster type" {vide "Introduction"); while north of Dun- 

 gannon, and further northward at Annaghone are Coal-measures 

 (Tykone Coal-fields). Near Cookstown and Coagh, and extend- 

 ing southward past Dungannon into the Co. Armagh, Trias (" Red 

 Free") is found. 



