246 On the Laurentian Rocks of Donegal and of other parts of Ireland. 
(1.) Lower Silurian Series—The schists, quartzites, and limestones which are 
represented on the geological maps as ranging in a south-westerly direction from 
the Atlantic coast, through Inishowen, to the shores of Lough Swilly, and re-appear- 
ing on the opposite coast, have been shown by Professor Harkness to be representa- 
tives of the beds of the Scottish Highlands south of the Caledonian Canal, 
which re-appear in Sutherland and Ross resting unconformably on the Cambrian 
sandstones and conglomerates.* Harkness accepts the interpretation, given by 
Sir R. I. Murchison, of the Lower Silurian age of these beds, and, therefore, arrives 
at the conclusion that the Donegal beds are likewise of Lower Silurian age. No 
one who has examined the Inishowen beds from the shores of L. Foyle to that of 
L. Swilly, and compares them with those of the North Highlands between 
Lochs Broom and Erribole can fail, as it seems to me, to recognise the soundness 
of this view, and it is one in which, with a personal knowledge of both districts I 
fully concur.t In both districts there is to be found a somewhat similar succession 
of quartzites, schists, and limestones, overlaid by a great series of chloritic and 
micaceous schists ; the chloritic schists being, however, much more fully developed 
in the North of Ireland, particularly in the counties of Derry and South-east Donegal, 
than in the Scottish Highlands. Making allowances, however, for changes due to 
geographical space, there is a remarkable similarity in the characters of the strata 
in the lower portion of the series in both districts; but the highly metamorphosed 
character of the limestones of Donegal, has, hitherto, rendered it impossible to 
obtain traces of fossil forms, such as have been recognised in the Durness limestone 
of Scotland. 
Tt is not my intention to give a detailed description of the Lower Silurian 
Series of Donegal after what has already been written. The general succession 
of the lower beds is shown in the tongue of land lying between Mulroy Bay and 
Lough Swilly, of which a generalized section is given.[ (Plate XX. Fig. 1.) 
These beds cross Mulroy Bay, and rise into the escarpment of Lough Salt, which 
terminates westward against the eneissose hills of Crockmore, south of Glen, where 
we obtained the clearest evidence of the unconformity of this series to that of the 
gneiss itself. The section here is as follows—in descending order :— 
Section or Lowsr Sriurian Bens. 
. Quartzites ; forming the ridge above the south shore of L. Salt (L. Salt Mountain 1,546 ft). 
. Beds of hornblendic diorite. 
Beds of quartzite and quvartz-schist. 
. Blue limestone—about 50 feet thick. 
. Beds of schist and quartzite. 
. Lower thin bedded limestone, about 100 feet thick, forming the escarpment S. of L. Greenan. 
7. Schists, &c. 
OO me wD 
* Quart. Journ. Geol. Soc. Vol. XVII. (1861). 
t In the spring of 1880, the author examined the district of the North Highlands, above referred to, in 
company with Mr. R. G. Symes, and under the guidance of Professor Geikie, Director of the Geological 
Survey of Scotland. An account of this visit is published in the Scientific Proc. Roy. Dub. Soc. Vol. III. 
{ This section was made in 1875, during a previous visit. 
