1 64 Notes on the Geological History of [Sess. 



might be formed in a volcanic vent, a view which has been set 

 aside by the recent identification of these rocks as lava. Re- 

 viewing the evidence available at the time of its publication, 

 the second edition of the Geological Survey Memoir of the 

 neighbourhood of Edinburgh is disposed to place the main 

 centre of volcanic activity in the vicinity of Colinton, persuaded 

 that in that locality the lavas attained their maximum thick- 

 ness. The fact that a great crustal displacement traverses the 

 country here affords some warrant for belief that a foundered 

 volcanic zone lay along that line, forming a ridge resembling, 

 it may be, Ontake in Japan, which has twelve extinct craters 

 along its summit. 



Briefly stated, disregarding details, the varied volcanic rocks 

 of the Pentland Hills — basalts, andesites, rhyolites, trachytes, 

 and tuffs — evidently originated from a series of vents of Lower 

 Old Eed Sandstone age, situated not far from the northern 

 limits of the hills. Breaking out on a lake floor, they built 

 up a thickness of some thousands of feet of rocks. Blackford 

 Hill basalt and andesite belong to the highest or latest mem- 

 bers of the series — that is to say, a bore put down in West 

 Mains Road would pierce successively every lava of the 

 series, assuming, for the purpose of illustration, that each 

 flow spread so far. 



One is tempted by the fascination of the subject to picture 

 to oneself the scene at the close of volcanic activity : an ele- 

 vated ridge, extending from the position of the Calton Hill 

 south-westwards beyond Currie, with its silent craters, and an 

 expanse of scarred lava plains stretching away to the south. 

 North of the ridge the scene may have been repeated, or an 

 intervening lake may have lain between it and the smoking 

 volcanoes of the Ochils and Fife. Conglomerates derived from 

 Silurian mountains in the south are found interbedded with 

 certain of the lava flows, implying that the Moorfoot-Cloich 

 area was at that time a high land suffering erosion. 



The period which followed is marked by earth movements, 

 accompanied by immense denudation, continued over a vast 

 interval of time. Evidently the volcanic ridge collapsed, for 

 successively, from north-east to south-west, lower and lower 

 divisions of the volcanic pile must have been exposed to 

 attack by atmospheric agents, which followed hard on crustal 



