DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 85 



as it's truncated strata rise high along its border, and look far over the low grounds on 

 every side, it must be regarded as a mere fragment of the original volcanic plain. It 

 may be described as an undulating tableland, which almost everywhere terminates in a 

 range of bold cliffs, but which, towards the centre and south, sinks gently into the basin 

 of Lough Neagh. The marginal line of escarpment, however, presents considerable 

 irregularity both in height and form, besides being liable to frequent local interruptions. 

 It is highest on the west side, one of its crests reaching at Mullaghmore, in county 

 Londonderry, a height of 1825 feet. On the north, it sinks down into the valley of the 

 Bann, east of which it gradually ascends, forming the well-known range of cliffs from the 

 Giant's Causeway and Bengore Head to Ballycastle. It then strikes inland, and making 

 a wide curve in which it reaches a height of more than 1300 feet, comes to the sea again 

 at Garron Point. From that headland the cliffs of basalt form a belt of picturesque 

 ground southwards beyond Belfast, interrupted only by valleys that convey the drainage 

 of the interior of the plateau to the North Channel. Above the valley of the Lagan the 

 crest of the plateau rises to a height of more than 1500 feet. 



Throughout most of its extent the basalt-escarpment rests on the white limestone or 

 Chalk of Antrim, beneath which lie soft Lias shales and Triassic marls. Here and there, 

 where the substratum of Chalk is thin, the action of underground water in the crumbling 

 shales and marls below it has given rise to landslips. The slopes beneath the base of the 

 basalt are strewn with slipped masses of that rock, almost all the way from Cushendall to 

 Larne, some of the detached portions being so large as to be readily taken for parts of 

 the unmoved rock. On the west side also, a group of huge landslips cumbers the 

 declivities beneath the mural front of Benevenagh. 



I have found some difficulty in the attempt to ascertain what ; was the probable form 

 of surface over which the volcanic rocks of this plateau began to be poured out. The 

 Chalk sinks below the sea-level on the north coast, but, in the outlier of Slieve Gallion, 

 three miles beyond the western base of the escarpment, it rises to a height of 1500 feet 

 above the sea. On the east side also, it shows remarkable differences of level. Thus, 

 below the White Head at the mouth of Belfast Lough, it passes under the sea-level, but 

 only 16 miles to the south, where it crops out from under the basalt, its surface is about 

 1000 feet above that level. If these variations in height existed at the time of the out- 

 pouring of the basalt, the surface of the ground over which the eruptions took place was 

 so irregular that some hundreds of feet of lava must have accumulated before the higher 

 chalk hills were buried under the volcanic discharges. But it seems to me that much of 

 this inequality in the height of the upper surface of the Chalk is to be attributed to 

 unequal movements since the volcanic period, which involved the basalt in their effects, 

 as well as the platform of Chalk below it. Had the present undulations of that platform 

 been older than the volcanic discharges, it is obvious that upper portions of the basalt- 

 series would have overlapped lower, and would have come to rest directly on the Chalk. 

 But this arrangement, so far as I am aware, never occurs, except on a trifling scale. 

 Wherever the Chalk appears, it is covered by sheets of the lower and not of the upper of 



VOL. XXXV. PART 2. M 



