468 Geikie — Volcanic Rocks of Great Britain. 



county of Kerry, numerous bands of ash — one of tliem reaching a 

 thickness of from 500 to 600 feet — stretch from mountain to moun- 

 tain under many hundred feet of overljdng sandstones and slates.^ 



Nor are traces of volcanic activity wanting in England during the 

 same great geological period. In Cornvpall and South Devon, Sir 

 Henry De la Beche recognised frequent proofs of contemporaneous 

 igneous action among the limestones and slates of the middle 

 Devonian series, and thence through 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 

 greenstone or other trap rock. The ash passes by insensible degrees 

 into the ordinary sedimentary strata of the series. Sometimes 

 it contains fossils, and in certain places it becomes so calcareous, and 

 so interlaced with bands of limestone, as to have been qnarried for 

 lime. The compact trap rocks associated with the ash bear evidence 

 of their contemporaneous origin in their frequently cellular and 

 pumiceous character.^ 



Carboniferous. — The base of the Carboniferous series in Cornwall 

 and South Devon is marked by the occurrence in it of sheets of 

 trappean ash and of crystalline amygdaloidal greenstone, similar 

 to the igneous masses among the neighbouring Devonian rocks. 

 The ash is sometimes coarse and full of fragments of cellular trap, as 

 in the conspicuous hill of Brent Tor. In describing the rocks of 

 that locality, Sir Henry De la Beche pointed out the remarkable 

 resemblance of the Brent Tor to a volcano, and the probability that 

 the ash and greenstone were erupted over the sea bottom, where 

 they became interstratified with the ordinary marine sediments.^ 



In the centre of England the well-known toad-stones of Derby- 

 shire indicate intermittent volcanic activity during the formation of 

 the Carboniferous Limestone. They consist of three principal beds 

 of trap, sometimes compact and dark, approaching basalt in texture, 

 but usually more earthy and highly amygdaloidal. These beds 

 average each about 60 or 70 feet in thickness, and preserve their 

 course for many miles between the strata of limestone. Mr. Jukes 

 has pointed out that each of them is probably the result of not 

 merely one eruption, but rather consists of different flows proceeding 

 from distinct vents, and uniting into one sheet along a common 

 floor.* 



Further north the counties of Durham and Northumberland are 

 traversed for many miles by interpolated sheets of dolerite, of which 

 the most important is known as the Great Whin Sill. It does 

 not appear that these masses have yet been investigated in such 

 detail as to indicate how far they may be actually contemporaneous 

 with the Carboniferous Limestone series in which they occur. 



Passing into Scotland, we find the Carboniferous formation of the 

 broad midland valley full of the most striking evidences of volcanic 



1 Mem. Geol. Surv. Ireland. Explanation to Sheet 184. See also Explanation to 

 Sheet 153, p. 18. 



2 De la Beche, Devon and Cornwall, pp. 51, 70. 



3 Ibid., p. 122. * Manual, p. 523. 



