CHAP. XXXVII 
THE ANTRIM PLA TEA U 
201 
the volcanic sheets is here 700 feet. 1 Many other displacements, 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. 
Besides actual dislocations, the Antrim plateau has undergone some marked 
subsidences of which the most notable is that of Lough Neagh. 2 
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 the volcanic outbursts 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 Irish 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 two and a half 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 the Chalk. Such 
variations appear to be mainly attributable to unequal erosion before the 
overflow of the basalts. So great indeed had been the denudation of the 
Cretaceous and underlying Secondary formations previous to the beginning of 
the volcanic eruptions, that in some places the whole of these strata had been 
stripped off the country, so that the older platform of Pakeozoic 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 basalts 
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. 2 The 
flints are generally reddened and shattery. The chalk itself lias been 
described as indurated, and its flints as partially burned 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. 4 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 doubt represents long-continued subserial 
weathering previous to the outflow of the basalt. Even, therefore, if there 
1 Explanatory Memoir of Slieets 7 and 8, Geological Survey, Ireland, by Messrs. Syn-.es, 
Egan, and M ‘Henry (1888), p. 37. 
2 These inequalities in the level of the base of the Antrim plateau will be more particularly 
discussed in Chapter xlix., in connection with the subsidences and dislocations which have 
affected the region since the close of the volcanic period. 
3 Portlock, Report on Geology of Londonderry, etc. (Geological Survey), p. 117. 
4 See Portlock, op. cit. p. 116. 
