254 On the Laurentian Rocks of Donegal and of other parts of Ireland. 
the shore of Kilkieran Bay. The section shows a thickness of these beds of about 
18,000 feet, and is illustrated in Figure 6, Plate X XI. 
The resemblance of these beds, both to those of Laurentian age in Donegal and 
in the Northern Highlands, is almost complete; and with the experience and 
knowledge gained since the Survey was finished in Galway, I can have no hesitation 
in referring them to the same period of geological time. 
Absence of Cambrian Beds in the North and West of Ireland.—Nowhere throughout 
Donegal, Mayo, Sligo, or Galway have we any evidence of the existence of Cambrian 
beds between the LowerSilurian and the Laurentian series. Considering the relations 
of these districts to those of the West and North Highlands, it may be supposed 
that Cambrian beds, had they been present, would have assumed the character of 
the rocks of this formation as they occur in the North-west Highlands of Scotland ; 
that is, the form of reddish sandstones and conglomerates, unconformable alike 
to the Laurentian beds below and the Lower Silurian beds above. But we fail to 
recognise any rocks having these characters and relations throughout the region 
referred to, in consequence of which there is here a double hiatus as compared with 
Scotland, except in those districts in Sutherlandshire, where the Lower Silurian 
beds rest directly upon the Laurentian. To this subject I shall have occasion 
to return presently (Page 256). 
(4.) Tyrone Rocks.—\ have now to refer to a district in which the relations of the 
beds are somewhat obscure, but which is mineralogically of great interest. It has 
recently been referred to by Dr. Hicks as being probably one of his numerous 
“ Pre-Cambrian ” areas.* It lies in county Tyrone, in the vicinity of Pomeroy ; and 
as the Geological Survey is still being carried on over the area, immediately to the 
north, and as the question regarding the relations of the rocks is still to a certain 
extent swb judice, I shall only briefly describe the locality here, in order to show 
exactly the conditions of the problem. 
The district in question forms a range of hills, having a general trend from west 
to east, of which Craigballyharky (771 feet) Caragrim (710 feet), Cregganconree 
(993 feet) and the Scalp (859 teet) are the most elevated prominences.t+ 
This range consists of granitic, pyroxenic, and felspathic rocks in great variety — 
chiefly of metamorphic origin, described by General Portlock,+ and more recently, 
and in great detail, by Mr. Nolan of the Geological Survey. Mr. Kinahan 
considers them of “Cambrian” age or “an older formation.”|| Along the south 
these old rocks are bounded by conglomerates of ‘‘ Lower Old Red Sandstone” age, 
but now generally recognised as representatives of the “‘ Dingle Beds,” lying at the 
* Proc., Geologists’ Association, Vol. VIL, No. 1, p. 28. 
+ Expl. Mem., Sheet 34 of the Geol, Survey, by Joseph Nolan (1878). 
{ Geol. of Londonderry, é&c., the term pyroxenic in here used to include rocks in which both hornblende 
hypersthene, augite, and diallage are prevalent, and appear to be associated with each other. 
§ Geol. Mag., April, 1879. The reader will find ample details in Mr. Nolan’s paper. 
|| Proc., Royal Irish Acad., Dec., 1880. 
