236 Reports and Proceedings — Geological Society of London. 



being the upper) form an irregular plateau, resting on rocks of 

 Carboniferous age ; no vents were observed ; but Mount Washburne, 

 a few miles distant, is a broken-down volcanic cone, from whicb 

 both trachyte- and basalt-flows (the latter the newer) have proceeded. 



2. " On the Microscopic Structure of Devitrified Rocks from 

 Beddgelert, Snowdon, and Skomer Island." By F. Eutley, Esq., 

 F.G.S. 



The first specimen described was found about a quarter of a mile 

 from Beddgelert, on the Capel Curig road. Examined microscopi- 

 cally, it showed traces of perlitic structure, with small spherulites, 

 both isolated and in bands, not exhibiting radial structure, but 

 apparently composed of very minute chlorite and a garnet, probably 

 spessartine. Hence the rock must be a devitrified obsidian or 

 pitchstone. The second specimen is a banded greenish-grey " fel- 

 stone," at Clogwyn du'r Arddu, of Bala age, which also has probably 

 been vitreoiis. The third specimen, from near Pont y Gromlech, is 

 a schistose felsitic rock. This was compared microscopically with 

 an obsidian from Hungary and a rhyolite from Gardiner's River 

 (N. America), and was shown to have been probably once a glassy 

 rock. In conclusion the author discussed the limits of the terms 

 felstone, rhyolite, trachyte, and obsidian. 



An appendix was added upon the microscopic characters of some 

 rocks from Skomer Island, off the coast of Pembrokeshire. These 

 were shown to be devitrified obsidians, some of them exhibiting 

 spherulitic and perlitic structures. A trachytic rock and a basalt 

 from the same locality were also described. 



3. "The Date of the last Change of Level in Lancashire." By 

 T. Mellard Reade, Esq., C.E., F.G.S. 



The author described some observations made by him at Blundell- 

 sands, on the coast of Lancashire, near Liverpool, according to which, 

 judging from the position of high- water mark, the land had gained 

 considerably upon the sea between 1866 and 1874. At one end of 

 a length of 350 yards, spring-tide high-water mark had receded 



15 yards, and at the other end 5 yards. The author estimated that 

 the deposit of sand that had accumulated in 8 years amounted to 

 an average of 10 yards wide and 2 yards deep. Allowing a further 

 depth of 1 yard for sand that may have been blown over the top, he 

 finds 10,500 cubic yards as the quantity of sand deposited in 8 ^^ears 

 on a shore-frontage of 350 yards, or 3-75 cubic yards per yard of 

 frontage per annum. Applying this unit of measurement to the 



16 miles of coast forming the western boundary of the deposit, he 

 gets 105,600 cubic yards as the quantity of sand annually moved ; 

 22 square miles of sand, 12 feet thick, give 272,588,800 cubic yards 

 of sand accumulated, which, divided by the annual quantity, will 

 give 2580 years as the age of the whole deposit of blown sand. The 

 author adduced other evidence in support of his view, and concluded 

 that if the last change of level in South-west Lancashire was a 

 downward one, it could not have taken place within 2500 years. 



