154 J. Nolan — Metamorphic and Intrusive Rocks of Tyrone. 



"In Slave Lake, in particular, which is 200 miles long, the 

 quantity of drift timber brought down annually is enormous." The 

 trees become water-logged and sink, and "the trunks gradually 

 decay, until they are converted into a blackish-brown substance 

 resembling peat, but which still retains more or less of the fibrous 

 structure of the wood ; and layers of this often alternate with layers 

 of clay and sand." The banks have " a remarkable horizontal 

 slaty structure," and along the Mackenzie "display almost every- 

 where horizontal beds of woody coal, alternating with bituminous 

 clay, gravel, sand, and friable sandstone." . . . "The Slave Lake 

 itself must, in process of time, be filled up by the matters daily 

 conveyed into it by the Slave Eiver." [Principles of Geology, 1868, 

 10th edit., vol. ii. p. 526.] 



To believe that this huge fragment of a large deposit belongs to 

 the Miocene, we have to suppose that it was deposited and all, ex- 

 cept this patch, denuded at a period when we have no record of 

 agents existing in our country capable of doing the one or the other, 

 and this in spite of lithological and palaeontological evidence which 

 directly connects them with a similar mass of strata not 80 miles 

 distani. 



III. — On the Metamorphic and Intrusive Rocks or Tyrone. 1 



By J. Nolan, M.R.I.A., etc. ; 



of H.M. Geological Survey of Ireland. 



THE rocks I propose to describe in this paper belong to that 

 great metamorphic series which occupies so large a portion of 

 the north of Ireland, extending over most of the counties Tyrone, 

 Londonderry, and Donegal. For the present I shall confine my 

 observations principally to that portion of them situated about the 

 central parts of Tyrone, from the vicinity of Omagh eastwards and 

 north-eastwards towards Slieve Gallion. This district has been 

 already ably described by General Portlock in his " Geological 

 Report on Londonderry and parts of Tyrone and Fermanagh " ; but 

 as, during the progress of my work for the Geological Survey, I have 

 had opportunities for a very detailed examination of it, I beg, 

 briefly, to offer some further remarks and conclusions. 

 The rocks in the district may be thus classified : — 

 First. — Schist and Gneiss. 



Secondly. — Green metamorphic rock, generally hornblendic or 



pyroxenic, passing in some parts into schist, and in others 



into granite. 



Thirdly. — Quartz Porphyry and Granite — metamorphic and 



intrusive. 



First. — Schist and Gneiss. — Little requires to be noticed of these 



rocks, as in this district they present but few special characteristics. 



They occupy a hilly tract of country, extending from near Carrick- 



1 This paper is published by permission of the Director of the Geological Survey 

 of Ireland, and of the Director- General. It was read before the British Asso- 

 ciation in Dublin, August, 1878. 



