150 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



liable to complete removal from wide tracts of country than older 

 formations which have been protected by having large portions of 

 their mass carried down by extensive faults and synclinal folds, and 

 by being buried under later sedimentary accumulations. We ought 

 not therefore to judge of the extent of the volcanic discharges 

 during Permian time merely from the small patches of lava and 

 tuff which have survived in one or two districts, but rather from 

 the number, size, and distribution of the vents which the work of 

 denudation has laid bare. 



And here let me remark that the evidence for the geological age 

 of some parts of the volcanic series now to be described is less 

 direct and obvious than most of that with which I have been 

 hitherto dealing. It consists of two kinds, (a) In the first of these 

 we find a series of lavas and tuffs regularly interstratified with the 

 red sandstones, which it is agreed to regard as Permian. The geolo- 

 gical horizon of these rocks is sufficiently clear. (6) Connected with 

 them are necks which obviously served as vents for the discharge of 

 the volcanic materials, and pierce not only the Coal Measures, but 

 even parts of the bedded lavas. So far there is not much room for 

 difference of opinion; but as we recede from districts where the 

 record is tolerably complete, we enter extensive tracts where only 

 the necks remain. All that can be positively asserted regarding 

 the age of these protrusions is that they must be later than the 

 rocks which they pierce. But we may inferentially connect them 

 with the series now under consideration by showing that they can 

 be followed continuously outward from the latter as one prolonged 

 group, having the same distribution, structure, and composition, and 

 that here and there they rise through the very highest part of the 

 Coal Measures. It is by inferences of this kind that I include as 

 relics of Permian volcanoes a large number of vents scattered over 

 the centre of Scotland, especially in the east of Fife. 



1. Bedded Lavas and Tuffs. — I shall first describe the volcanic 

 chronicle as it has been preserved in the south-west of Scotland, 

 where the existence of Permian volcanoes in Britain was first re- 

 cognized.^ The volcanic rocks in that district rise from under the 

 central basin of red sandstone, which they completely enclose. 

 Their outcrop at the surface varies up to about a mile or rather 

 more in breadth, and forms a pear-shaped ring, measuring about nine 

 miles across at its greatest width. 



^ See explanation of Sheet 14, Geol. Surv. Scotland (1869), p. 22 ; and the 

 paper in the G-eol. Mag. for 1866 already cited. 



