Professor Geikie on the " Pitchstone " of Eskdale. 237 



quantities of molten rock should have ascended so many 

 fissures over such an extended area without somewhere flow- 

 ing out at the surface. Yet to the east of the line of great 

 basalt plateaux there are no proofs of any such superficial 

 outflow having occurred, unless certain erupted sheets in the 

 Lanarkshire and Stirlingshire coalfields can be so regarded. 



That an enormous amount of the general denudation of 

 this country has taken place since the extravasation of the 

 dykes is shown by innumerable instances in which the dykes 

 run along the crests of hills and cross important valleys. It 

 is manifest that the hills could not have had their present 

 form and altitude, otherwise the basalt, if it reached the sur- 

 face along their crests, would have poured down their sides. 

 It is equally obvious that where a dyke runs as a bold pro- 

 minent rib down either side of a valley, the valley, at least 

 at its present depth and form, must have been eroded since 

 the dyke was erupted. By evidence of this kind it can be 

 shown that to a large extent the existing valleys of this 

 country have been excavated since Miocene times.* In this 

 vast denudation it is possible tliat sheets of basalt that were 

 formerly spread over various parts of the country as outflows 

 from the dykes, have been wholly removed. But when we 

 consider the durability of this rock in massive sheets, we are 

 led to doubt whether any extensive superficial outpourings 

 ever took place except over the great western plateaux.f 



During many years of exploration among the Tertiary 

 volcanic rocks of this country, I have been often struck with 

 the remarkable infrequence of traces of any true associated 

 volcanic vents, such as those which occur so abundantly in 

 connection with the Palaeozoic igneous rocks. Over the wide 

 area of the mainland of Scotland and northern England — an 

 area thousands of square miles in extent — where the basalt 

 dykes can now be traced at the surface, not a single 



* See Quart. Jour. Geol. Soc, 1871, p. 310., and Trcms. Roy. Soc. Edin., 

 loc. cit. 



+ That veteran geologist, Dr Ami Boue (who still lives to take an interest 

 in Scottish geology), realised the difficulties presented by the dykes, but con- 

 ceived that the melted rock might have been supplied from above by super- 

 ficial sheets of moving lava, though he admitted that this idea involves the 

 necessity of enormous denudation.— ^sg'ia'sse Geologiquc sur VEcosse, p. 272. 



