The whole of Cardiganshire is GEOLOGICALLY included in the great slate and shale tract of South Wales, and produces in different places roofing-slates of various qualities, flooring-stones, &c., besides an excellent hard kind of building-stone, of which the houses of Aberystwith exhibit good specimens, and a kind of sandstone of a fine grain, found in Penbryn parish, which is little inferior to freestone, but of a darker colour. The stratification is in most places very irregular. The grey mountain trap rocks, which produce the excellent building-stones above-mentioned, extend in prominent lines from north-east to south-west, the broadest constituting ranges of hills abounding with mineral veins: their stratification is in some places very irregular, while in others it presents regular quadrilateral columns of excessively close texture. The stone is the most extensively quarried on Llanwenog Hill, to the west of Lampeter; near Llêchrhŷd; and near Penbryn; forming good ashlars, tombstones, troughs, and rollers. Connected with the rocks are beds of indurated schist, porphyroids, &c. One range of these hills extends the whole length of the county, from the banks of the Dovey to the west of Machynlleth, through the mining districts, to Plumstone mountain in Pembrokeshire. The roofing-slates, which vary in colour from grey to blue, are sometimes inter-stratified with argillaceous schistus of a softer texture, commonly called shale, which soon decomposes when exposed to the action of the atmosphere: this shale is also found singly in various places. The best blue argillaceous slates for roofing are quarried and dressed at Ynys Hîr, near Cors Vochno: and various other quarries of the same material occur along the seacoast; but none of any extent have been opened in the interior, and the slates are far inferior in size and quality to those of Carnarvonshire. The strata of blue schist also, in numerous places, afford excellent building-stones, of which the county gaol and church tower at Cardigan are good specimens: the blue colour of this stone, when neatly worked, gives it a very pleasing appearance. Large veins of a hard and glossy white spar, called hungry spar rider, frequently occur among the other strata. The strata nearest the surface, in the south-western part of the county, consist of the clay marl which is sometimes used as a manure: the higher layers of it are brown and of an inferior quality; the lower are blue and richer, resting immediately on the schistose strata above described. The eastern and northern boundary of this tract of clay marl, crossing the Teivy into county Cardigan from the vicinity of Penboyr in Carmarthenshire, curves north-westward towards the mouth of the Aëron, forming on the land side part of the periphery of a circle, within which is included the whole south-western part of the county. Between Llanina and New-Quay the cliff overhanging the sea is composed, for the most part, of this marl, which there varies in depth from six to twenty feet and upwards.