1 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



Moel Siabod, in North Wales, also tlie result of their labours on the 

 Geological Survey. They show that a series of sandstones and con- 

 glomerates, with some beds of purple and blue slates, and occa- 

 sionally trap rocks, about 3000 feet thick, constitute the base of 

 that part of Wales. These are known as the Barmouth and Harlech 

 sandstones, constituting the land which borders the coast, at and 

 between those places, and forming the lower part of an irregular 

 dome-shaped mass, dipping, where covered by other rocks, beneath 

 the next or trappean group. This, with some flexures belonging to 

 it, is well known to you from the descriptions of Professor Sedgwick, 

 constituting his ' Great Merioneth Anticlinal.' 



The trappean group is so named from containing igneous rocks, 

 some felspathic, others hornblendic, wdth beds of 'ash,' probably 

 ejected into the air from volcanic vents and, falling in water, arranged 

 like ordinary detritus by tides and currents, as noticed in the address 

 of last year. The authors separate this group, to the whole of 

 which they assign a thickness of 15,000 feet, into two divisions, the 

 lower containing blue and grey slates and flagstones, and known as 

 the Lingula beds, an abundance of that shell, with some other fossils, 

 being discovered in them. In the upper division are many inter- 

 stratified beds of black slate, often occurring as irregular and lenti- 

 cular masses, and graduating into ' ash.' Lingulee and graptolites 

 are found in these beds, though not very abundantly. 



Upon these deposits rests the Bala group, estimated at 9000 feet 

 thick. This is divided into a lower series, composed of black slates, 

 of variable thickness, fine-grained and brittle, the true lamination 

 being often entirely concealed by cleavage and numerous joints ; — 

 and an upper series formed of a grey arenaceous slate, often passing 

 into a hard splintery grit. In the lower part of this latter division 

 there are one or two beds of trappean ash, and in its centre occurs 

 the limestone so well known as the Bala limestone, celebrated for its 

 organic remains. Sometimes, but very rarely, there is a band of 

 limestone in its upper part, of which the Hirnant limestone is an 

 example. 



The authors state that almost all the igneous rocks of the trap- 

 pean group are contemporaneous with the beds amid which they are 

 found, the ash beds formed from igneous materials arranged in 

 water, — the gradual passage above and beneath the more solid trap- 

 pean rocks showing that " its exhibition was intimately connected 

 with the commencement and end of the igneous action which pro- 

 duced them." 



It may not be here out of place to mention that the progress of 

 the Geological Survey has shown, that after the dip of the Bar- 

 mouth and Harlech sandstones north-westward beneath the trappean 

 group of Snowdon and its associated mountains, they rise again, 

 though with diminished thickness, in the line of country passing 

 across the Llanberis lakes, resting upon those highly cleaved purple 

 beds, so extensively employed for roofing-slates, in the valleys of 

 Nant Francon, Llanberis, and Llanllyfni. These slates repose on 

 and are interstratified with sandstones and conglomerates, mingled 



