Geikie — Volcanic Rocks of Great Britain. 471 



tain recognisable plants of Miocene species. This vast depth of old 

 lavas and tuffs points to a lengthened continuance of volcanic 

 activity along the north-western margin of our country — an activity, 

 however, marked by prolonged periods of repose, as the Scuir of 

 Eigg,^ and the coal and shales of Mull, sufficiently prove. For 

 magnitude, alike in thickness and extent, these Tertiary volcanic 

 rocks surpass those of any of the other formations in our area. But 

 I believe that these masses, vast though 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 towards 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 : I am not aware that any 

 single one can be followed completely across the island, though the 

 well known Cleaveland dyke in the north of England runs for at 

 least sixty miles, cutting in its course Carboniferous, Permian, 

 Triassic, Liassic, and Oolitic rocks till it reaches the sea on the coast 

 of Yorkshire, at a distance of more than 200 miles from the nearest 

 point where the sheets of Miocene trap are now visible. In Ber- 

 wickshire 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 Ayrshire 

 they become still more numerous, traversing the coal-field and 

 altering the coal seams ; in Arran and Cantyre their number still 

 increases ; until, after a wonderful profusion of them in Islay and 

 Jura, they reach the great volcanic chain of the Hebrides. From 

 their manifest intimate connection with that chain, from the fact 

 that they cut through all the formations they encounter up to and 

 including the Chalk, and that they cross faults of every size that 

 may lie in their way, I regard these dykes as of Tertiary age. If 

 this inference is sustained, as I have little doubt it will be, by a 

 more detailed investigation of the north-western districts, it presents 

 us with striking evidence of the powerful activity and wide range of 

 the volcanic forces in our country during the Miocene period. With 

 these dykes (to which further allusion will be made in the sequel), 

 and the Tertiary igneous masses from which they proceed, the record 

 of volcanic action in Britain appears to close. 



This brief reference to the proofs of contemporaneous volcanic 

 eruptions during the growth of the successive geological formations 

 in the British Islands may suffice to indicate the wide area of re- 

 search which here presents itself to geologists. Let me allude to 

 one or two portions of this broad field, which seem to me worthy of 

 special notice. 



One of the first features to arrest attention is the singular per- 



^ See Scenery of Scotland, viewed in conaection with its Physical Geology, p. 278 



