466 GeiUe — Volcanic Rocks of Great Britain. 



a broader view of the whole subject, we may to some extent trace 

 the progress of volcanic action over the whole country. Again, the 

 rocks may be examined, irrespective of the formations to which they 

 belong, as repositories of data respecting the phenomena of volcanos. 

 They may be studied as chemical or mineralogical compounds, and 

 compared or contrasted with the products of modern volcanos. 

 When, moreover, we reflect how many of these igneous masses must 

 have consolidated on the floor of the sea, and how rare are the 

 opportunities of investigating the progress of an active submarine 

 volcano, we perceive that an attentive study of our own volcanic 

 rocks may even elucidate some of the less observable features of 

 modern volcanic action. Or these igneous masses may be examined 

 with the view of ascertaining how far volcanic activity may influence 

 submarine life. Thus, in some of our geological systems, among 

 the Silurian rocks of Wales, for example, or the Carboniferous lime- 

 stone group of Fife and the Lothians, many instructive sections 

 occur where an abundant series of crinoids, corals, brachiopods, and 

 other organisms, is gradually or suddenly enveloped in a mass of 

 tuff. Other instances likewise abound in which a suite of fossils 

 may be found slowly struggling through the upper part of a bed of 

 tuff, until the ashy sediment dies away, and the fossils gather to- 

 gether into a bed of limestone. Even among the coal seams and 

 ironstones of Scotland such intimate relations to contemporaneous 

 volcanic action may be traced. 



I purpose, at this time, to point out, by one or more illustrations 

 from each of our geological formations wherein volcanic rocks occur, 

 how varied and long-continued has been the progress of volcanic 

 action in these islands. I shall offer, in conclusion, some sugges- 

 tions as to phases of the subject which seem to me deserving of more 

 special study than has yet been accorded to them. 



Lower Silurian. — The oldest recognisable volcanic rocks in this 

 country belong to the lower Silurian period. They are best dis- 

 played in North Wales, where, as was shown long ago by Sir 

 Roderick Murchison, they rise into conspicuous ranges of hills. 

 Two principal epochs of eruption have been detected by Professor 

 Ramsay and his colleagues of the Greological Survey. One of these 

 occurred during the deposition of the Llandeilo rocks, and is indi- 

 cated by the igneous rocks of Aran Mowddwy, Cader Idris, Arenig, 

 and Moelwyn : the other is marked by those of the Snowdon dis- 

 trict, which lie among the Bala beds. These volcanic rocks consist 

 partly of massive sheets of felstone, varying in texture and colour, 

 and partly of thick accumulations of tuff or ash. The former are 

 true lava, flows, the latter point to frequent showers of volcanic dust, 

 and to the settling of such dust and stones on the sea bottom, where 

 they mingled with the ordinary sediment, and with shells, corals, 

 and other organisms. Some of these ashy deposits attain a great 

 thickness. Thus, at Cader Idris, "they are about 2,500 feet thick, 

 the accumulated result of many eruptions." Northwards this mass 

 thins entirely away, and the ordinary sedimentary strata take its 

 place. Equally local are the massive beds of felstone which repre- 



