174 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



comparison. One formidable obstacle lies in the difficulty in 

 determining chronological equivalents in separated groups ot rock. 

 Geologists have tried to determine whether the. volcanoes of some 

 particular period or region were in any way connected with such 

 geological changes as extensive plication, dislocations of the crust, or 

 elevation of mountain -chains. In regard to the volcanic history of 

 Britain, various possible relations of this kind may be suggested ; 

 but nothing quite satisfactory has been determined. 



The only fact that can be certainly affirmed is that our volcanoes 

 have been active on sinking rather than on rising areas. We 

 usually associate modern volcanic action with elevation, and there 

 are certainly abundant proofs of such elevation around active or 

 recently extinct volcanoes. It may be, however, that such uprise 

 is merely a temporary incident, and that if we could survey the 

 whole geological period of which human history chronicles^so small 

 a part, we might find that subsidence, and not upheaval, is ulti- 

 mately the rule over volcanic areas. Be this as it may, there can 

 be no question that with the one solitary exception of the Tertiary 

 volcanoes, which were terrestrial and not submarine, all our vents 

 were carried down and eventually buried under aqueous sediments. 

 That they are now above sea-level is of course due to long sub- 

 sequent movements not immediately connected with their original 

 formation. The amount of subsidence which followed on a volcanic 

 episode was sometimes enormous, even within the same geological 

 period, as one may see by observing the prodigious piles of sedi- 

 mentary material heaped over the Arenig lavas and tuffs. I do not 

 wis-^h to maintain that the downward movement was necessarily a 

 consequence of the volcanic ejections, for we know that it took 

 place over tracts remote from the centres of eruption. But I have 

 sometimes asked myself whether it was not possibly increased as a 

 sequel to vigorous volcanic action ; whether, for instance, the great 

 depth of the older Palaeozoic sedimentary rocks in this country as 

 compared with their development in other countries, and notably in 

 Scandinavia, may not have been due to an acceleration of subsi- 

 dence consequent upon volcanic action. 



8. It appears to me that two types of volcanic manifestation may 

 be recognized in the geological history of our region. In the first 

 place there is the ordinary type of volcanic vents, with the local and 

 limited outflowing lavas and often abundant ashes, as in the great 

 series of Palaeozoic eruptions. In the second place there is the type 

 in which the formation of abundant fissures, and the uprise of lavas 



