County Donegal on 1837
The Floetz limestone-field, which occupies the central plain of Ireland, extends over the borders of this county from Bundoran, where the limestone cliff rises to the height of 100 feet over the Atlantic, ten miles north-east to Ballintra, where the extreme edge of the stratum is perforated by a subterraneous river.
Limestone gravel is also found along the flanks of the primitive district as far as some miles north of Donegal town, and to the presence of this valuable substance may be chiefly attributed the cultivation which distinguishes this part of the county from the sterile tract that separates it from the basin of the Foyle.
From the mountains of Barnesmore, north, the whole formation of this county, with the exception of the transition tract along the basin of the Foyle, is primitive.
The prevalent rocks are granite and mica slate, passing into gneiss, quartz slate, and clay slate.
The granite is a coarse granular syenite, the detritus of which gives a strong reddish tinge to the sands washed down by the streams that traverse it. It occurs supporting flanks of mica-slate along the whole line of mountains from Loch Salt to Barnesmore.
On the eastern flanks of this range the mica slate passes into greywacke, which forms the substratum of the valley of the Foyle: the same rock occurs over the lower parts of Inishowen, and also appears on the southern side of the range near Donegal town.
Granular limestone is found in beds throughout the whole mountain district in great quantity and variety of colour, as among various other indications, grey at Malin Head; greyish-blue at Loch Salt; fine granular, pearl-white, pearl-grey, flesh-red, and bright bluish-grey, at the marble hill near Muckish; yellowish-white, greyish-white, and rose-red, at Ballymore, pearl-white and pale rose colour at Dunlewy, under Erigal; pearl-grey in extensive beds at the head of the river Finn; and greyish fine blue at Killybeggs. Siliciferous, magnesian, and marly limestone also occur in various parts of the baronies of Inishowen and Raphoe, with a remarkable steatite near Convoy, on the Deele, which cuts under the knife like wood, and is used by the country people for the bowls of tobacco-pipes.
Beds of greenstone and greenstone porphyry are sometimes found resting on the deposits of granular limestone, and occasionally on the mica slate and granite, and the dikes from which these originate may be seen traversing the primitive rock at Horn Head and Bloody Foreland.
Among the rarer minerals occurring in this remarkable region are columnar idocrase, malacolithe, epidote, and essonite (cinnamon stone), from a bed of mica slate in the Rosses, and from the bar of the Gweebarra river; garnet in hornblende slate over the marble of Dunlewy ; and cherry-red garnet from Glanties: also plumbago from the shore of Ardes; copper pyrites from Horn Head; lead earth and iron ochre from Kildrum, in Cloghanealy; pearl-grey and yellowish-white porcelain clay from Aranmore Island; potter's clay from Drumardagh, on Loch Swilly; iron pyrites from Barnesmore; lead ore from Finntown, Letterkenny, Glentogher, and various other places; and pipe-clay from Drumboe, near Stranorlar.
The white marble of Dunlewy, near the mountain Erigal, is stated to be of an excellent quality, and its bed very extensive; it has been traced over a space of half a mile square, and is so finely granular, that it may be employed in the nicest works of sculpture.
'Its texture and whiteness,' says Mr. Griffith, 'approach more to those of the Parian than of the Carrara marble. It is very well known that perfect blocks of the Carrara marble are procured with great difficulty, and I firmly believe that the marble of Dunlewy is free from mica, quartz grains, and other substances interfering with the chisel, which so frequently disappoint the artists who work upon the marble from Carrara.
A large supply of fine siliceous sand was formerly drawn from the mountain of Muckish by the glass-houses of Belfast, and considerable quantities have been of late exported to Dunbarton for the manufacture of plate and crown glass: the sand is rolled down the hill in canvas bags.