TRANSACTIONS OF THE SECTIONS. 51 



as probably of Oolitic age. But more recent investigations in Antrim, Mull, and 

 Eigg, have convinced me that in these districts, and probably also in Skye, the 

 great basaltic plateaux which form so conspicuous a feature in the scenery of om* 

 north-western sea-board, date from tertiary times. From Antrim northwards 

 through the inner Hebrides and the Faroe Islands to Iceland there is a broken 

 chain of volcanic masses, part, and not improbably the whole, of which are of 

 Miocene age. In Ireland sheets of dolerite and basalt, in all 500 or 600 feet 

 thick and some 1200 square miles in extent, repose directly upon an eroded sur- 

 face of chalk. In Mull, similar plateaux, overlaid with masses of porphyrite and 

 trach}i;e-]ike rocks, attain a united thickness of more than 3000 feet, yet at their 

 base they contain recognizable plants of Miocene species. This vast depth of old 

 lavas and tufls points to a lengthened continuance of volcanic activity along the 

 north-western margin of our coimtry — an activity, however, marked by prolonged 

 periods of repose, as the Scuir of Eigg and the coal and shales of Mull sufficiently 

 prove. These masses, vast thougli they be, are by no means the only, if they are 

 indeed the chief, relics of Tertiary volcanic action in Britain. If, starting from the 

 basaltic plateaux of the north of Ireland or of the inner Hebrides, we advance to- 

 wards the south-east, we soon observe that an endless number of trap-dykes, 

 striking from these plateaux, extends in a south-easterly direction athwart our 

 island. The south-western half of Scotland and the northern parts of England are, 

 so to speak, ribbed across with thousands of dykes. These are most numerous 

 near the main mass of igneous rock, whence they become fewer as they recede 

 towards the North Sea. Usually a dyke cannot be traced far. In Berwickshire 

 and the Lothians, these E. and W. or N.W. and S.E. dykes, often less than half a 

 mile long, are well shown ; in AjTshire they become still more numerous, tra- 

 versing the coal-field and altering the coal-seams ; in An-an .and Cantyre their 

 number still increases, until, after a wonderfid profusion of them in Islay and Jura, 

 they reach the great volcanic chain of the inner Hebrides. From their manifest 

 intimate connexion with tliat chain, from the fact that they cut through all the 

 formations they encomiter up to and including the chalk, and that they cross faults 

 of every size that maj' lie in their way, I regard these dykes as of tertiary age. If 

 this inference is sustained, as I have little doubt it wiU be, by a more detailed 

 investigation of the north-western districts, it presents us with striking evidence 

 of the powerfid activity and wide range of the volcanic forces in our country 

 dming the Miocene period. With these dykes, and the Tertiary igneous masses 

 from which they proceed, the record of volcanic action in Britain appears to 

 close. 



Let nie now allude to one or two portions of this broad subject which seem to 

 me worthy of special notice. One of the first features to arrest attention is the 

 singular persistence of volcanic phenomena in a limited area. Take, as an illustra- 

 tion, the neighbourhood of Edinburgh within a radius of ten miles from the town. 

 First and oldest comes the long range of the Pentland and Braid hills, consisting 

 of a mass of bedded igaieous rocks in a middle series of the Old Bed Sandstone. 

 These old lavas reach a thiclmess of 4000 or oOOO feet. Next in chronological 

 order are the Calton Hill and lower portion of Arthur's Seat, which mark the con- 

 tinuance of volcanic action into the Lower Carboniferous period. The carboniferous 

 rocks for miles around these hills are full of the traces of contemporaneous volca- 

 noes, sometimes in the form of sheets of tuff marking the occmTcnce of little 

 detached tufi-cones, sometimes in wider areas of tuff, basalt, and dolerite, where a 

 group of minor volcanic vents threw out showers of ash and streams of lava. To 

 the east rise the isolated Garlton Hills, which date from before the carboniferous 

 limestone ; westwards, scores of little basaltic crags and rounded tuff-hills mark 

 out the lower carboniferous volcanoes of Linlithgowshire. To the north, the end- 

 less crags, hills and hiUocks of the Fife coast contain the record of many eruptions 

 from the middle of the calciferous sandstones high up into the carboniferous lime- 

 stone gTOup. Even the coal-measures of that county are pierced with intrusive 

 bosses of trappean agglomerate, which indicate the position of volcanic vents, pos- 

 sibly of Permian age. The same or a more recent date must be assigned to the 

 later unconformable agglomerate and basalt of Arthin-'s Seat. Nor is this the 

 whole. Latest of all come innumerable trap-dykes, running with a prevalent east 



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