Geikie — Volcanic Rocks of Great Britain. 469 



activity. From the very bottom of the series up to at least the top 

 of the Carboniferous Limestone group volcanic rocks of many 

 varieties abound. In the West, great sheets of different porphyrites, 

 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 

 Eenfrewshire hills, to the banks of the Irvine in Ayrshire, and 

 thence westwards by the Cumbrae Islands and Bute, to the south of 

 Arran.^ In the eastern districts, instead of such wide-spread sheets 

 of volcanic rock, the Carboniferous series includes hundreds of 

 minor patches of tuff, dolerite, basalt, and porphyrite. The area of 

 the Lothians and Fife seems to have been dotted over with innumer- 

 able 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, a hundred 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 wonderfully exhibited than in the coalfield of Dairy in 

 the northern part of Ayrshire. The black-band ironstone of that 

 district appears to have been deposited in hollows between mounds 

 and cones of volcanic tuff, sometimes 600 feet high, round and over 

 which the later members of the lower Carboniferous formation were 

 deposited. Hence the shafts of the pits are sometimes sunk for 100 

 fathoms through the tuff, and at that depth mines are driven hori- 

 zontally through the volcanic rocks to reach the ironstone beyond. 

 In other districts the interstratification of beds of ash and sheets of 

 basalt and dolerite amongst highly fossiliferous limestones and shales 

 present many points of interest. In this respect the range of the 

 Linlithgowshire hills is specially deserving of study. 



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 different parts of the sea bottom. 

 In the county of Limerick masses of trap 1200 and 1300 feet thick, 

 Avith well marked ashy iaterlacings, lie among the limestones.^ 



Permian.— 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 



1 The trap-rocks forming these hills are interstratified in their upper portion with 

 the Carboniferous Limestone. Their base rests sometimes on a set of marls, shales 

 and cement- stones, and sometimes on a thick group of red sandstones. These strata 

 contain Carboniferous plants ; even thin coal seams lie among their higher members, 

 and although the red sandstones have been hitherto generally called Old Red Sand- 

 stone, it is not unlikely that they may require to be relegated wholly to the lower 

 portion of the Carboniferous series. 



2 See Maclaren's Fife and the Lothians. Mem. Geol. Survey, Geology of 

 Neighbourhood of Edinburgh. Trans. Roy. Soc, Edin., Vol. xxii., p. 644. 



3 See Mem. Geol. Surv., Ireland. Explan. to Sheets 143, 144, 163, and 154; also 

 Jukes' Manual, p. 325. 



