The Carboniferous rocks of Shropshire possess several peculiar features; and no member of the series shows these in a greater degree than the Millstone-grit, as seen on Sweeney Mountain, near Oswestry. Before discussing these features, it will not be amiss to detail briefly the character of this rock in other localities. It varies considerably in different places. For instance, in the Forest of Dean, it is a hard intractable rock. Such it is also in Glamorgan and Monmouthshire, where it is often seen in place under the Coal-measures, or in boulders on the hillsides. Varteg Hill, near Pontypool, is a most characteristic spot for it; that hillside being covered with masses of grit, of all sizes and shapes. These masses arenot unfrequently wholly made up of water-worn quartz-pebbles, occasionally as large as a hen's egg, in a cement of sand and decomposed felspar. And although hundreds of houses, with their garden-walls, have been built of them, yet considerable areas of these boulders remain. Very large blocks of this rock may be seen on the southern flanks of the Black Mountains, Caermarthenshire, above the village of Cross-Inn. The Millstone-grit of the South-Welsh Coal-field, which goes by the name of ‘Farewell Rock’— from the fact that the miner on striking it bids farewell to coal, possesses the valuable property of being able to resist successfully for a length of time the action of most intense heat, and for this reason the ‘hearths’ of iron-furnaces are constructed of it.