86 DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



the two groups into which the Antrim basalts are divisible. We have actual proof of 

 considerable terrestrial disturbance, subsequent to the date of the formation of the 

 volcanic plateau. Thus, near Ballycastle, a fault lets down the basalt and its Chalk 

 platform against the crystalline schists of that district. On the east side of the fault, the 

 Chalk is found far up the slope, and circling round the base of the beautiful cone of 

 Knocklayd — an outlier of the basalt which reaches a height of 1695 feet. The amount 

 of vertical displacement of the volcanic sheets is here 700 feet.* Many other displace- 

 ments, as shown by the mapping of my colleagues in the Geological Survey, have shifted 

 the base of the escarpment from a few inches up to several hundred feet. 



It is evident, therefore, that the present position of the Chalk platform is far from 

 agreeing with that which it presented to the outflow of the sheets of basalt. But, on the 

 other hand, there can be no doubt that its surface at the beginning of th evolcanic out- 

 bursts was not a level plain. It was probably a rolling country of low bare chalk-downs, 

 like parts of the south-east of England. The Chalk attains its maximum thickness of 

 perhaps 250 feet at Ballintoy. But it is liable to rapid diminution. On the shore at 

 Ballycastle, about 150 feet of it can be seen, its base being concealed ; but only 2\ miles 

 to the south, on the outlier of Knocklayd, the thickness is not quite half so much. On 

 the west side of the plateau also, there are rapid changes in the thickness of Chalk. Such 

 variations appear to be mainly attributable to unequal erosion before the outflow of the 

 basalts. So great indeed had been the denudation of the Cretaceous and underlying 

 Secondary formations previous to the beginning of the volcanic outbursts, that in some 

 places the whole of these strata had been stripped off the country, so that the older 

 platform of Palaeozoic or still more ancient masses was laid bare. Thus, on the west side 

 of the escarpment, the basalt steals across the Chalk and comes to rest directly upon 

 Lower Carboniferous rocks. 



The authors who have described the junction of the Chalk and basalt in Antrim have 

 generally referred to the uneven surface of the former rock as exposed in any given section. 

 The floor on which the basalt lies is remarkably irregular, rising into ridges and sinking 

 into hollows or trenches, but almost everywhere presenting a layer of earthy rubbish made 

 of brown ferruginous clays, mixed with pieces of flint, chalk, and even basalt.t The 

 flints are generally reddened and shattery. The chalk itself has been described as 

 indurated, and its flints as partially burnt by the influence of the overlying basalt. But 

 I have not noticed, at any locality, evidence of alteration of the solid chalk, except where 

 dykes or intrusive sheets have penetrated it.| There can be no doubt that the hardness 

 of the rock is an original peculiarity, due to the circumstances of its formation. The 

 irregular earthy rubble, that almost always intervenes between the chalk and the base of 

 the basalt, like the " clay with flints " so general over the Chalk of southern England, no 



* Explanatory Memoir of Sheets 7 and 8, Geological Survey, Ireland, by Messrs Symes, Egan, and M'Henby 

 (1888), p. 37. 



+ Portlock, Report on Geology of Londonderry, &c. (Geological Survey), p. 117. 

 X See Portlock, op. cit., p. 116. 



