Page:A Treatise on Geology, volume 1.djvu/146

 130 lowest (probably hypozoic) strata visible in Wales. Then follow sandstones and conglomerates—black shales of Bangor—other sandstones and conglomerates interstratified with trappean rocks—then the fine purple slates of Llanberis and Nant Francon. In all this vast series no fossils have been discovered.

Above are sandstones and slates beneath the trappean group of Snowdon, on the summit of which are the fossils of Bala. These sandstones, however, are much better seen in the section from Cader Idris to Moel Siabod. They are thus described by Sir H. De la Bechet :—

"A series of sandstones and conglomerates, with some beds of purple and blue slates, and occasionally trap rocks, about 3000 feet thick, constitute the base of that part of Wales. These are known as the 'Barmouth and Harlech Sandstones,' and they are brought up by an axis of elevation called by Sedgwick the 'Great Merioneth Anticlinal.'

"A trappean group succeeds above, containing contemporaneous igneous rocks, some felspathic, some hornblendic, with beds of 'ash,' probably ejected into the air, and falling in water, arranged like ordinary detritus by tides and currents, about 15,000 feet thick. This is formed in two divisions; the lower, containing blue and gray slates and flagstones, is known as 'the Lingula beds,' from the abundance of that shell which, with some others, occurs in them. In the upper division are many interstratified beds of black slate, often occurring as irregular and lenticular masses, and graduating into 'ash.' Lingula and graptolites occur in these beds, though not abundantly."

Upon these rests the Bala group. This is in accordance with the early general views of Sedgwick. Thus by combining the information which these researches yield we have strata of slate, sandstone, conglomerate, and trappean ash, many thousand yards in thickness, below the lowest band of what was considered, in 1836,