VOLCANIC PEAKS OF THE GRAMPIANS. 297 



surface on weathering, is found in the Bala beds, about a mile N.W. 

 of the summit of Snowdon, and closely corresponds with obsidians 

 from volcanic districts in its fluxion structure. 



A dark-grey felstone, with a fissile structure, is seen between 

 Pont-y-Gromlech and Gorphwysfa, so that it might be termed a felsite 

 schist, or an indurated volcanic ash, unless examined under the micro- 

 scope ; but its microscopic texture corresponds with many obsidians 

 and rhyolites, and is probably to be regarded as a devitrified obsi- 

 dian, because the base is entirely devoid of crystalline structure. 



Skomer Island, off the coast of Pembrokeshire, contains volcanic 

 rocks which are associated with strata belonging to the Llandeilo or 

 to the Bala series. They are banded and spherulitic, and what were 

 once obsidians are now felstones. They closely resemble those of the 

 Yellowstone district of the United States. With the felstones occur 

 other rocks, classed by Mr. Rutley as basalt, and as quartz oligoclase 

 trachyte. 1 



The Silurian period, comprising the Wenlock and Ludlow rocks, 

 appears to have been one in which volcanic action was inter- 

 mitted. 



Devonian and Old Red Sandstone Volcanoes. 



Old Eed Sandstone and Devonian Volcanoes. With the Old 

 Red Sandstone and Devonian period, eruptions began again with 

 great vigour, but their locality is removed from Wales and the 

 Lake district. Interbedded volcanic rocks abound in North Devon, 

 West Somerset, and South Devon ; but northward there is no trace 

 of an Old Red Sandstone volcano till we reach the south and centre 

 of Scotland. 



Volcanic Peaks of the Grampians. Professor Judd has drawn 

 attention to a series of granite masses which burst through the 

 Cambrian strata ; and, forming the axis of the Grampian chain, 

 extend from the Ross of Mull in the S.W. in a N.E. direction to 

 Peterhead. The largest of these masses are Cairngorm, Ben Nevis, 

 and Ben Cruachan. Professor Judd remarks, that when the granite 

 boss is greatly denuded, as in the Ross of Mull, the rock exposed 

 is a typical granite ; but where it rises into lofty peaks, it becomes 

 more and more hornblendic, and graduates externally into a felsite, 

 which is more or less porphyritic. Everywhere, when in contact 

 with stratified rocks, the granite sends veins into them, which de- 

 monstrates that it was sufficiently heated to be rendered fluid by the 

 removal of the rock pressure upon it ; and the existence of the 

 fissures now filled with granite is an evidence of the influence which 

 such alteration of pressure exercised. Professor Judd's section of 

 Ben Nevis may be regarded as demonstrating that lava and ashes 

 were poured out from old volcanoes, of which the granite bosses of 

 the Grampians are the cores. The Ben Nevis rocks consist at the 



1 Rutley : "Devitrified Rocks from Bedd-Gelert and Snowdon," Q. J. G. S.. 

 xxx vii. p. 403. 



