SILURIAN PERIOD.
Wigtownshire.—The possibility of glacial action so far back as the Silurian age has been suggested. In beds of slate and shales in Wigtownshire of Lower Silurian age Mr. J. Carrick Moore found beds of conglomerate of a remarkable character. The fragments generally vary from the size of one inch to a foot in diameter, but in some of the beds, boulders of 3, 4, and even 5 feet in diameter occur. There are no rocks in the neighbourhood from which any of these fragments could have been derived. The matrix of this conglomerate is sometimes a green trappean-looking sandstone of exceeding toughness, and sometimes an indurated sandstone indistinguishable from many common varieties of greywacke.[155]
Ayrshire.—Mr. James Geikie states that in Glenapp, and near Dalmellington, he found embedded in Lower Silurian strata blocks and boulders from one foot to 5 feet in diameter of gneiss, syenite, granite, &c., none of which belong to rocks of those neighbourhoods.[156] Similar cases have been found in Galway, Ireland, and at Lisbellaw, south of Enniskillen.[157] In America, Professor Dawson describes Silurian conglomerates with boulders 2 feet in diameter.
Arctic Regions.—The existence of warm inter-glacial periods during that age may be inferred from the fact that in the arctic regions we find widespread masses of Silurian limestones containing encrinites, corals, and mollusca, and other fossil remains, for an account of which see Professor Haughton’s geological account of the Arctic Archipelago appended to McClintock’s “Narrative of Arctic Discoveries.”[158]