TRANSACTIONS OF THE SECTIONS. 49 



thus constantly undergoing, always in one and tlie same vessel, a round of regularly 

 recurring changes of state of combination, by wbich it passes, iirst from the state of 

 protocliloride to that of protoxide, next from the state of protoxide to that of a 

 higher oxide, capable of liberating chlorine from hydrochloric acid, then bacli again 

 to the state of protocliloride, and so on continually. 



aEOLOGT. 



Address by the President, Auchibald Geikie, F.R.S., F.G.S. 



Among the Lower Silurian, the oldest recognizable volcanic rocks in this country, 

 two principal epochs of eruption have been detected by Professor llarasay and his 

 colleagues of the Geological Survey. One of these occurred during the deposition 

 of the Llandeilo rocks, and is indicated by the igneous rocks of Aran Mowddwy, 

 Cader Idris, Arenig, and Moelwyn ; the other is marked by those of tlie Snowdon 

 district, which lie among the Bala beds. These volcanic roclfs consist partly of 

 massive sheets of felstone, varving in texture and colour, and partly of thick accu- 

 mulations of tuff or ash. The former are true lava-flows, the latter point to fre- 

 quent 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 sliells, corals 

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

 at Cader Idris, they are abou.t 2500 feet tliick, the accumulated result of many 

 eruptions. Northwards this mass thins entirely away, and the ordinary sedimentary 

 strata take its place. Equally local are the massi-\-e beds of felstone which repre- 

 sent the submarine lava-flows of the time. Sometimes they still preserve the 

 slaggy vesicular character which marked their surface when the meltiKl rock was 

 in a state of motion along the sea-bottom — an evidence of the existence and posi- 

 tion of true submarine volcanoes during the Lower Silurian period in Wales. In 

 the lake district, similar proofs of volcanic action have been found among the lower 

 Silurian rocks of that region. In Scotland, no ^-ery distinct traces of volcanic acti- 

 ^dty have yet been detected among rocks of the lower Silurian age. In the Lower 

 Silurian rocks of the south-west of Ireland, beds of ash and felstone are interstra- 

 tified, resembling in general character and mode of occurrence those of Wales. In 

 Wales, volcanic action does not appear to have outlasted tlie Lower Silurian period ; 

 but in Ireland, among the headlands of Kerry, massive sheets of ash are intercalated 

 in grits and slates, which, from their fossils, have been assigned to the age of the 

 Wenlock series. 



The Old Red Sandstone of the southern half of Scotland abounds in igneous 

 rocks, from the base of the series to the top. In its lower band lie the chains of 

 the Sidlaw and Ochil Hills, and many detached masses scattered over the lowlands 

 along the southern flank of the Grampians. These are composed of different fel- 

 stones and porphyrites, with interbedded sheets of tutt" trappean conglomerate, and 

 sandstone, stretching in the Ochil and Sidlaw range for sixty or seventy miles, and 

 risino- here and there to heights of 2000 feet above the level of the sea. This 

 group of hills contains some of the thickest masses of trappean rock in the country. 

 In what seems to be a middle portion of the formation conies the group of the 

 Pentland Hills, consisting of long massive beds of trap, like the difterent varieties 

 in the Ochils, with intercalations of tutf, conglomerate, and sandstone, the wlicje 

 reaching a thickness of fully 5000 feet. In Ireland also the Old Tied Sandstone 

 furnishes evidence of active volcanic vents. Nor are traces of volcanic activity 

 wanting in England dm-ing the same great geological period. In Cornwall and 

 South Devon frequent proofs have been recognized of contemporaneous igneous 

 action among the limestones and .slates of the Middle Devonian series, and thence 

 throuoli the Upper Devonian into the lower part of the Carboniferous group. These 

 consist in frequent bands of trappean ash, and of crystalline amygdaloidal and 

 vesicular green.stoue or other trap. The ash passes by insensible degrees into the 



1867. 4 



