84 EIGHTH REPORT — 1838. 



not to the transition series ; and also tliat the red conglomerate beds 

 of Cahirconree, and Carrantoohill mountains, together with the coarse 

 red slate of which Tornies and Glena mountains at Killarney are com- 

 posed, belong to the old red sandstone series. 



It is to be observed, that the schistose strata belonging to the se- 

 condaiy formations of the south of Ireland are much more compact than 

 those of the northern districts, and hence we find the quartzy slates 

 and sandstones of the old red sandstone series have assumed the form 

 of coarse clay slates and quartzy rocks ; and also the dark gray car- 

 boniferous slates of the south of the county of Cork, which contain 

 Orthocerata, Calamites, &c., have assumed the character and fissile struc- 

 ture of ordinary roofing clay slate ; and several extensive slate quarries 

 have been opened in different parts of the district, but these slates are 

 not found to be of a durable nature. 



On a small Tract of Silurian Rocks in (he County of Tyrone. By 

 Captain Portlock, F.G.S. 



Captain Portlock remarked that he first recorded the existence in 

 the County of Tyrone of rocks of the Silurian system, in the 1st volume 

 of the Ordnance Survey Memoir of the County of Derry, and that he 

 considers this to be the first authenticated case of their occurrence in 

 Ireland, though there is little doubt that they also exist in Kerry and 

 other counties. The tract in question is small, extending only over a 

 few miles of surface in the eastern portion of Tyrone ; it rests upon 

 granitic and other primary crystalline rocks, and is succeeded by rocks 

 partly belonging to the old red sandstone, and partly to the carbonife- 

 rous system, by which it is completely detached from other rocks of a 

 similar epoch. Apparently it has been raised up from its original level 

 by the intrusion or eruption of the granitic mass, a movement which 

 must have occurred prior to the deposition of the more recent rocks, as 

 they exhibit no appearance of disturbance. The portion of the Silu- 

 rian system, here exhibited, appears capable of sub-division. The lower 

 or gritty slate is manifestly a fragmentary rock, and remarkable for 

 containing great numbers of a brachiopodous bivalve, which is either 

 identical with, or very similar to, Orthis grandis of Murchison. Above 

 this is a black, smooth schist, occasionally slightly calcareous, and some- 

 times thinly laminated by calcareous spar. This is the depository of the 

 Graptolites (^Lomatoceras, Bronn), which are abundant. 



The upper part of the Silurian district is a more decided slate, and 

 abounds in Trilobites of the genera, Calymene, Asa.phus, Cryptolithus, 

 (Green,) Trinucleus, (Llwyd and Murchison). Illanus? perovalis, 

 Murchison, and what Captain Portlock is inclined to believe the true 

 Isotelus, besides some species of doubtful genera. This district will be 

 fully illustrated in one of the earliest forthcoming parts of the Ordnance 

 Memoir, and these and other fossils, such as Orthocerata, Belleroplion, 

 Lingula, etc. figured. 





