50 REPORT— 1867. 



ordinary sedimentary strata of the series, sometimes containing fossils, and in certain 

 places so interlaced with bands of limestone as to have been quarried for lime. 



The base of the Carboniferous series in CornNvall and South Devon is marked by 

 the occurrence of ash and crystalUne amygdaloidal greenstone similar to the ig-neoiis 

 masses in the neighbouring Devonian rocks. In the centre of England the -well- 

 known toad-stones of Derbyshire indicate intermittent volcanic activity during the 

 formation of the carboniferous limestone. They consist of three principal beds of 

 trap, averaging each about 60 or 70 feet in thickness, preserving their course for 

 many miles between the strata of limestone, probably, as pointed out by Mr. Jukes, 

 the result not merely of one eruption, but rather of different flows from distinct 

 vents, and uniting into one sheet along a common floor. Passing into Scotland, we 

 find the carboniferous formation of the broad midland valley full of the most stri- 

 king evidences of volcanic activity. In the west, gi-eat sheets of different porphy- 

 rites, with interbedded tuffs, sandstones and conglomerates, lie in the lower part 

 of the formation, and rising in broad masses bed above bed, form that conspicuous 

 chain of terraced heights which stretches from near Stirling through the range of 

 the Campsie, Kilpatrick, and Eenfi-ewshire hills, to the banks of the Irvine in 

 Ayi-shire, and thence westward by the Cumbrae Islands and Bute, to the south of 

 Ai'ran. In the eastern districts, instead of such -widespread sheets of volcanic rock, 

 the Carboniferous series includes hundreds of minor patches of tuff', dolerite, basalt, 

 and porphjTite. The area of the Lothians and Fife seems to have been dotted over 

 with innmnerable little volcanic vents, breaking out and then disappearing one 

 after another during the lapse of the Carboniferous period up to at least the close 

 of the carboniferous limestone. The very limited area occupied by the erupted 

 material is often remarkable. A mass of ash 100 feet thick or more may be found 

 intercalated between certain strata, yet at a distance of a mile or two the same 

 strata may show no trace of any volcanic material. Nowhere is this feature more 

 wonderfullj' exhibited than in the coal-field of Dalrj' in the northern part of Ayr- 

 shire. The black-band ironstone of that district appears to have been deposited in 

 hollows between mounds and cones of volcanic tuft', sometimes 600 feet high, round 

 and over which the later members of the Lower Carboniferous formation were de- 

 posited. Hence the shafts of the pits are sometimes sunk forlOO fathoms through 

 the tuff; and at that depth nrines are driven horizontally through the volcanic rocks 

 to reach the ironstone beyond. The great carboniferous limestone series of Ireland 

 contains evidence that here and there, at various intervals during its formation, 

 minor volcanic vents were active on dift'erent parts of the sea-bottom. 



Among the Permian sandstones of the south-west of Scotland there occur some 

 interesting proofs of contemporaneous volcanic action. In Nithsdale, and still more 

 conspicuously in the centre of the Ayrshire coal-field, these sandstones contain 

 towards their base a thick group of dark reddish-brown amygdaloidal porphyrites 

 and tuft's. Connected -with these rocks are numerous bosses of a coarse volcanic 

 agglomerate, which descend vertically through the coal-measures, altering the coal. 

 They are the "necks" or orifices from which was ejected the volcanic material 

 which now forms a conspicuous range of rising grounds overlying the heart of the 

 coal-basin of Ayrshire. 



The New Red Sandstone series of Devonshire, in the neighbourhood of Exeter, 

 furnishes clear proofs of volcanic activity. Sheets of a dark reddish-brown fel- 

 spathic rock, sometimes compact or porphyritic, but usually of scoriaceous character, 

 are intercalated among the lower parts of the Red Sandstone series of that neigh- 

 bourhood. Sir Henry De la Beche, who described these igneous roclis many years 

 ago, noticed that the more compact portions, instead of extending horizontallj' as 

 beds among the sedimentary strata, descend vertically through them, as if these 

 detached parts marked the site of some of the orifices whence the melted lava was 

 rupted. 



The series of successive volcanic phenomena, which may thus be traced through 

 the palaeozoic rocks of the British Islands up to the New Red Sandstone, is now 

 abruptly brolten. I am not aware of any satisfactory proofs of contemporaneous 

 volcanic rocks among the secondary rocks of Britain, save in the Red Sandstone of 

 Devonshire just referred to. FolloM-ing a suggestion of Prof. Edward Forbes, I 

 fonnerly regarded the great ti-appean masses of Skye and the other western island.? 



