70 Geological Society: — 



ritic or spherulitic, which appear to throw some light on the epoch 

 at which these structures have heen set up. In a black porphy ritic 

 obsidian is a crystal which the author believed to be olivine. Zirkel 

 has already noticed the occurrence of this mineral in a trachyte. The 

 structure of some of the above indicates that it is extremely difficult 

 to draw hard and fast lines between trachytic rhyolites and fel- 

 stones. A tuff containing fragments of a rhyolite, some perlitic, was 

 also described. The spaces included within the boundary of some of 

 these perlitic cracks exhibit depolarization and sometimes inter- 

 ference-crosses. The author considered these to be the result of 

 strain in contraction, and connected with incipient crystallization. 



Andesites, from two localities in the northern part of the Yellow- 

 stone district, were also described. 



In an appendix Mr. Eccles briefly described the geology of the region 

 from which the above specimens were collected, referring for greater 

 detail to the memoirs of Dr. Hayden and his fellow workers. In the 

 Yellowstone-Park region trachyte and obsidian (the latter being the 

 upper) form an irregular plateau, resting on rocks of Carboniferous 

 age. Xo vents were observed ; but Mount Washburne, a few miles 

 distant, is a broken-down volcanic cone, from which both trachyte- 

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



2. "On the Microscopic Structure of Devitrifled rocks from Bedd- 

 gelert, Snowdon, and Skomer Island." By E. B-utley, Esq., E.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 appa- 

 rently composed of very minute chlorite and a garnet, probably spes- 

 sartine. Hence the rock must be a devitrifled obsidian or pitchstone. 

 The second specimen is a banded greenish-grey " felstone," at Clog- 

 wyn du'r Arddu, of Bala age, which also has probably been vitreous. 

 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, tra- 

 chyte, 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 devitrifled 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., E.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 



