/ 
256 On the Laurentian Rocks of Donegal and of other parts of Ireland. 
evidence towards the solution of the question will probably be obtained upon the 
completion of the survey over the eastern and southern parts of Londonderry now 
in progress. * 
It has been suggested that these rocks are of “Cambrian age.” In this view I 
cannot concur, on the ground that if they were of this age they ought to bear some 
resemblance to the Cambrian beds of the West Highlands of Scotland. To these 
they have not the remotest resemblance ; and, on a general review of the evidence, I 
feel disposed to refer them to an “ Upper Laurentian” stage not rising to the surface 
in Donegal owing to the overlap of the Lower Silurian beds, but emerging from 
under these beds along their southern border.—(See Map on Plate XXII.) 
REASONS FOR THE ABSENCE oF CAMBRIAN Breps In tHe Nort aAnp 
West or IRELAND. 
In another place I have endeavoured to account for the absence in the north 
and west of Ireland of those great beds of red sandstone and conglomerate which 
are interposed between the Lower Silurian quartzites and limestones of the North 
Highiands of Scotiand, and the Laurentian gneiss of the same region, and which 
form so conspicuous a physical feature in West Highland scenery. According to 
the views referred to, I have supposed that during the Cambrian period there 
existed an old Archean ridge lying in a north-east and south-west direction over 
the region now occupied by the Grampian Mountains on the north and the 
Donegal, Sligo, Mayo and Galway Highlands on the south. This ridge, having 
been unsubmerged, formed a tract of land of crystalline rocks—on either side of 
which the Cambrian beds were deposited—those on the north-west side, in the 
waters of an old lake (as suggested by Professor Ramsay), those on the opposite 
side (including the Cambrian beds of the east of Ireland of Wales and Shropshire, 
&c.), in the waters of the sea.t Owing, therefore, according to this view, to the 
Donegal district, as well as those of Mayo, Sligo and Galway being in a condition 
of dry land during the Cambrian period no strata were deposited over these areas, 
which were, on the other hand, exposed to a process of denudation during the 
Cambrian period. 
* Mr. Nolan in a letter, dated 8th June, says, “I have nothing to say against the view that the highly 
metamorphic rocks of Tyrone may be of Laurentian age. They graduate into gneiss their area, but their 
relations to the great mass of schists on the north is (as you say) obscure.” 
{ For the reasons on which these views are based the reader may refer to my paper ‘On the two 
types of Cambrian beds of the British Islands (the Caledonian and Hiberno-Cambrian), and the conditions 
under which they were respectively deposited,” Proc. Brit. Assoc. (1881.) Trans of Section C.; also 
Quart. Journ. Geol. Soc. Lond., No. 412, 1882. 
