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CHAPTER XVIII. 



THE HISTORY OF VOLCANIC ACTIVITY IN BRITAIN. 



THE British region was greatly disturbed by volcanic pnenomena 

 during the whole of the Primary period, and during the earlier por- 

 tion of the Tertiary period volcanoes were active in Western Scot- 

 land ; but except at its commencement in the Triassic rocks of 

 the south-west of England, the whole of the Secondary epoch was 

 free from volcanic outbursts. During a large part of the time re- 

 corded by the Primary strata, the British region was a volcanic archi- 

 pelago throwing out vast quantities of volcanic ashes and lavas, 

 which not infrequently form much of the thickness of some of the 

 older Primary rocks. 



Evidence for the Former Existence of Volcanoes. The ex- 

 istence of these volcanoes is affirmed chiefly on the evidence of the 

 volcanic ashes and agglomerates j for although intrusive lavas may 

 be injected in sheets into strata and between them, long subsequent 

 to their deposition, no source for volcanic ashes is conceivable except 

 the throat of an active volcano. The very sites where such ancient 

 centres of volcanic activity stood are usually unknown, and often 

 only inferred, with more or less uncertainty, from dykes and bosses of 

 volcanic rock. But although long since levelled, like Graham Island, 

 and having the throats or necks covered up by later deposits, denu- 

 dation, which has exposed the old strata forming the structure of the 

 country, yields a history of volcanic action, which we propose to 

 trace. 



In the following pages we begin with the most ancient eruptions, 

 and follow the outbursts of volcanic action during successive periods of 

 geological time. If there be any general differences between modern 

 and ancient volcanic effects, it is in part attributable to the circum- 

 stance that the modern phenomena best known to us are such 

 as happen on land subaerial eruptions while the evidence of 

 the ancient volcanoes is chiefly gathered from subaqueous lava 

 streams, and submarine beds of ashes. Much of the modern andesite, 

 trachyte, basalt, and other melted rock has been exposed by 

 eruption, and indurated at the surface ; much of the ancient 

 porphyrite, felsite, and greenstone was solidified under the pressure 

 of seas, and decomposed and indurated by prolonged infiltration. 



