DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 73 



be inferred. At least, we know, that many dykes do not reach the present surface, and 

 that those which do have shared in the enormous denudation of the surrounding country. 

 That even in the same dyke the lava rose hundreds of feet higher in one place than at 

 another is abundantly proved. When, however, we consider the vast number of dykes 

 that now come to the light of day, and reflect that the visible portions of some of them 

 differ more than 3000 feet from each other in altitude, we can hardly escape the conviction 

 that it would be incredible that nowhere should the lava have flowed out at the surface. 

 Subsequent denudation has undoubtedly removed a great thickness of rock from what 

 was the surface of the ground during older Tertiary time, and hundreds of dykes are now 

 exposed that originally lay deeply buried beneath the overlying part of the earth's 

 crust through which they failed to rise. But some relics, at least, of the outflow of lava 

 might be expected to have survived. I believe that such relics remain to us in the great 

 basalt-plateaux of Antrim and the Inner Hebrides. These deep piles of almost horizontal 

 sheets of basalt, emanating from no great central volcanoes, but with evidence of many 

 small local vents, appear to me to have proceeded from dykes that reached the highest 

 level, and from which orifices, communicating with the surface of the ground, allowed the 

 molten material to flow out in successive streams with occasional accompaniments of 

 fragmentary ejections. The structure of the basalt-plateaux, and their mode of origin, 

 will form the subject of the next division of this paper. . 



We can hardly suppose, however, that the lava flowed out only in the western region 

 of the plateaux. Probably it was most frequently emitted and accumulated to the 

 greatest depth in that area. But over the centre of Scotland and north of England there 

 may well have been many places where dykes actually communicated with the outer air, 

 and allowed their molten material to stream out over the surrounding country. The 

 disappearance of such outflows need cause no surprise, when we consider the extent of the 

 denudation which many dykes demonstrate. I have elsewhere shown that all over Scot- 

 land there is abundant proof that hundreds and even thousands of feet of rock have been 

 removed from parts of the surface of the land since the time of the uprise of the dykes.* 

 The evidence of this denudation is singularly striking in such districts as that of Loch 

 Lomond, where the difference of level between the outcrop of the dykes on the crest of 

 the ridges and in the bottom of the valley exceeds 3000 feet. It is quite obvious that, 

 had the deep hollow of Loch Lomond lain as it now does in the pathway of these dykes, 

 the molten rock, instead of ascending to the summits of the hills, would have burst out 

 on the floor of the valley. We are, therefore, forced to admit that a deep glen and lake- 

 basin have been in great measure hollowed out since the time of the dyke. If a depth of 

 many hundreds of feet of hard crystalline schists could have been removed in the interval 

 there need be no difficulty in understanding that by the same process of waste, many 

 sheets of solid basalt have been gradually stripped off the face of central Scotland and 

 northern England. 



* Scenery of Scotland, 2d edit. (1887), p. 149. But see the remarks already made (p. 55) on the curious coincidence 

 sometimes observable between the upper limit of a dyke and the overlying inequalities of surface. 



