DURING THE TERTIARY PERIOD IN" THE BRITISH ISLES. 89 



lowest sheet of the upper group, and may be seen resting directly on the zone of grey and 

 red tuffs. It is about 60 or 70 feet thick ; and, while perfectly regular in its columnar 

 structure at the Causeway and the " Organ," assumes further eastward the confusedly 

 starch-like arrangement of prisms already referred to. But, in the great cliff section of 

 the " Amphitheatre," the more regular structure is resumed, the bed swells out to about 80 

 feet in thickness, and columns of that length run up the face of the precipice, weathering 

 out at the top into separate pillars, which, perched on the crest of an outstanding ridge, 

 are known as the " Chimneys." The basalt-beds that succeed the lowest one are each 

 only about 15 to 20 feet thick. 



Between the successive sheets of the upper basalts thin seams of red ferruginous clay, 

 though, as I have said, less frequent perhaps than in the lower group, continue to show 

 that the intervals between successive eruptions were of sufficient duration to admit of 

 considerable subserial decay of the surface of a lava before the outflow of the next bed. 

 Occasional thin layers of tuff also, and even of pisolitic iron-ore, have been observed 

 among these higher basalts. But the most interesting and important intercalations are 

 inconstant seams of lignite. One of the most conspicuous of these lies immediately 

 above the basalt of the " Causeway," where it was long worked for fuel, and was found 

 to be more than 6 feet thick. But it is quite local, as may be seen at the " Organ " over 

 which it lies, with a thickness of only 12 inches, rapidly dying out so as to allow 

 the basalts above and below it to come together. The removal of the upper portion 

 of the basalts prevents us from carrying the volcanic history of the Irish plateau 

 further. 



It is obvious that nowhere in Antrim does any trace exist of a central vent or cone 

 from which the volcanic materials were discharged. There is no perceptible thickening 

 of the individual basalt sheets, nor of the whole series in one general direction, in such a 

 manner as to point to the site of some chief focus of eruption. Nor can we place reliance 

 on the inclination of the several parts of the plateau. I have pointed out that the varying 

 dip of the beds must be attributed mainly to post-volcanic movements, or at least to 

 movements which, if not later than all the phases of volcanic action, must have succeeded 

 the outpouring of the plateau-basalts. There has been a general subsidence towards the 

 central and southern portions of the plateau, and this movement has no doubt given rise 

 to the hollow that is now occupied by Lough Neagh. But nowhere in the depression is 

 there any trace of the ruins of a central cone or focus of discharge. 



The Antrim plateau, in these respects, resembles the others. But it differs from them 

 in one important particular. It has nowhere been disrupted by huge eruptive bosses of 

 younger rocks, such as have broken up the continuity of the old lava-fields further north. 

 Yet it is not without its memorials also of these younger protrusions. It has some feeble 

 representatives of the great acid bosses of the Inner Hebrides, and it contains not a few 

 excellent examples of true volcanic vents. To these fuller reference will be made in later 

 pages. 



2. Mull. — This plateau, besides the island of Mull, embraces a portion of Morven, 



