Donegal Geology

Granite forms the summit of all the mountains, and with the new red sandstone, rests on a substratum of limestone mostly of the primitive formation and containing no organic remains, although secondary limestone abounds in several parts. The limestone is found through all the level districts near the sea and elsewhere, and in the mountains forming the manors of Burleigh and Orwell. On the eastern shore of Lough Swilly, and in some other parts of Ennishowen, is found a species of calcareous argillite, having the appearance of grey limestone, but containing too much silex to burn freely. Round Carndonagh, in the same barony, is a dark blue limestone of superior quality.

Many species of valuable marble have been discovered. One of these, of a pure white, free from flaws or discolouration, and capable of being raised in blocks of any dimension at a trifling expense, has been found in the Rosses;, but the want of roads, though the quarries are at a short distance from the sea, prevents its exportation. Grey and black marble of very fine quality have also been found. Little advantage has hitherto been derived from any of the other mineral productions. Lead ore has been discovered in several places in the barony of Boylagh; in the river flowing from the mountain of Killybegs; on the surface near the western shore of Loughnabroden; at the foot of the Derryveagh mountains; in the Barra river; in Arranmore and other parts of the Rosses; and at Kieldrum, in the barony of Kilmacrenan, where there is a considerable deposit of ore collected for a lead-work which was carried on a few years since, but discontinued as being unprofitable from the want of experienced miners. Copper ore and iron pyrites may be traced in Errigal and Muckish mountains, and detached masses are found in several of the mountain streams and near Ballyshannon. Both these ores are abundant; and in several other parts the numerous vitriolic springs indicate larger deposits. Iron ore abounds in several parts.

As long as fuel could be procured from the forests of Donegal, Derryveagh, Slievedoon and Kilmacrenan, the mines were wrought and the ore smelted. The remains of bloomeries are often met with in the mountains and the foundations of forges near some of the rivers. Manganese is also abundant. Coal appears in a thin seam at Dromore, on the shore of Lough Swilly, and indications of it are frequent in Innishowen, but no attempts have yet been made to raise it. The same remark applies to steatite or soap-stone, here called "camstone," though found in abundance in all the mountains of Kilmacrenan and Bannagh: it is mostly of a bright sea-green colour. At Drumarda, on the shores of Lough Swilly, on Tory island, and in the Rosses, are extensive beds of potter's clay, which is used in a small degree in manufacturing coarse pottery. Pipe clay and other kinds of useful clays are found frequently, but little used. Silicious sand of a very superior kind is abundant at Lough Salt, and in the Ards, whence considerable quantities are exported for the manufacture of glass. Excellent slates are raised near Letterkenny, Buncrana, and in some other places.

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