266 



KNOWLEDGE 



[Decembeb 1, 1898. 



VOLCANOES OF THE NORTH. 



By Grenville A, J. Cole, m.r.i.a., fg.s., Professor of 

 Genlofiy in tJie Royal College of Science for Ireland. 



THE north-east corner of Ireland is eminently a 

 plateau country. When we enter Belfast Lough 

 from the sea, the irregular rounded hills of Down 

 find a contrast on the western side in the broad - 

 backed braes of Antrim. Dark cliffs of basalt 

 can be seen high upon the slopes, with here and there a 

 gleam of chalk beneath them. One or two deep valleys 

 have been cut through the plateau by the streams that 

 seek the sea ; but the general crest is level, some one 

 thousand one hundred feet above the water, until the 

 whole mass dies away into a series of rounded domes, far 

 away towards Moira in the south. 



The conspicuous black scarp runs round the coast to 

 Garron Point ; it is broken on the back of the ancient 

 gneiss of Torr ; and then it reappears, in its fullest gran- 

 deur, between Ballycastle and the Giant's Causeway. We 

 may follow it yet further, to the grim cliffs above the 

 lowland of Lough Foyle ; and then we may trace these 

 southward up the Eoe, to the noble heights above Dun- 

 given and the tableland at Moneymore. Within this 

 circuit of one hundred and fifty miles, the country is 

 uniformly covered with basaltic rocks. They dip down 

 towards the low-lying basin of Lough Neagh, but form 

 rapidly rising moorlands as we move again outward from 

 the water. Even on the western shore of the lake, 

 where their width is only some five miles, the basalts find 

 room for the production of the characteristic uplands, 

 clothed with gorse and heather. 



The plateaux thus cover almost all the County of Antrim, 

 and an important part of the County of Londonderry ; but 

 the scarped nature of their outer edge shows that they 

 must have formerly extended further. On the east, the 

 flat top of Scrabo Hill, near Newtownards, recalls the 

 features of the plateaux : and inspection shows that we 

 have here a thick mass of basalt, protecting the soft red 

 sandstones of the district. This hill is nine miles from 

 the main scarps above Belfast. On the west, again, there 

 is a remarkable outlier on the northern summit of Slieve 

 Gallion, one thousand five hundred feet above the sea, 

 from which the hillside falls rapidly on all sides. We look 

 away from it westward, across a wild country, worn out of 

 the older rocks, and can picture the basalt as stretching 

 on in old times, until it met the rim of its basin in the 

 very heart of Donegal. 



It is little wonder that such broad expanses of uniform 

 rock, lying in beds, tier upon tier, with an obvious tendency 

 to weather out as plateaux, were compared by many older 

 geologists with regularly stratified aqueous deposits. The 

 type of scenery common in County Antrim is thus repeated 

 among the limestone hills of Sligo, a district of inland 

 scarps and massive tablelands : and most of us are familiar 

 with such features in the stratified Pennine Chain of 

 England. Werner, reasoning from the isolated sheets of 

 basalt in central Germany, asserted that such rocks were 

 precipitated from solution in water ; and his views obtained 

 a remarkable hold upon men who were content to make 

 theories, rather than to imdertake laborious observations. 

 These " Neptunian " doctrines were part of a system which, 

 as Lyell quaintly remarks, " had not the smallest 

 foundation, either in Scripture or in common sense,"* 

 and were refuted by Werner's French contemporaries, 

 Guettard, Faujas de St. Fond, and Desmarest. Faujas, 



• ■■ Principles of Geology," Vol. I. (1830), p. 69. | 



in his fine folio work, attributes much of his information 

 to an elderly cleric, the Abb6 de Mortesagne, whose 

 enthusiastic and picturesque letters are printed in full. In 

 another letter we find M. Ozy, a chemist of Clermont- 

 Ferrand, attributing his own enlightenment as to the 

 volcanic nature of his country to the visit of " Olzendorff," 

 an Englishman, and " Bowls," an Irishman, who came 

 out in 1750 to study the lead mines of Auvergne. May we 

 not presume that the " M. Bowls " was acquainted with 

 Antrim and the Giants' Causeway, and found in the 

 perfectly preserved craters round the Puy de Dome the 

 verification of opinions formed in Ireland '? 



The matter has more interest than would at first appear ; 

 for the earliest printed appreciation of the volcanic origin 

 of the Irish basalts seems to be contained in the second 

 edition of a highly speculative work, by John Whitehurst, 

 published in 178C, Whether " Olzendorff" or " Bowls " 

 was the direct ijistructor of M. Ozy, the views propounded 

 by them from the summit of the Puy de Dome were 

 extremely novel in 1750. t The " Irlandois" was probably 

 the William Bowles who wrote a treatise on Spain in 

 1776, and whose mineral collection is known to have been 

 sold in 1830. 



The story of the struggle against the Wemerians, and 

 of the ultimate triumph of the supporters of volcanic 

 action, is well told by Portlock. ' The Liassic shale of 

 Portrush in Ireland has been baked by intrusive sheets of 

 dolerite, and has come to resemble the compact basalt 

 of the district. Its fossiliferous character made the 

 Wemerians haU it as a basalt containing marine shells, 

 and as an obvious proof of their contentions. Kirwan, who 

 established the first important mineral collection in Dublin, 

 supported this unhappy view. Playfair published the 

 true explanation in 1802, before he had visited the district ; 

 but the error lingered on for another fifteen years. Even 

 now, when the igneous origin of the plateau-basalts is 

 everywhere accepted, questions arise as to the vent or 

 vents from which such broad masses were erupted. 



There is no doubt that the great mass of the basalts of 

 north-eastern Ireland were poured out as lava-flows upon 

 a terrestrial surface. Despite later faults and dislocations, 

 the relation of the lower streams to this old land-surface 

 can again and again be seen. In the beautiful sections 

 along the Antrim coast, some of which are naturally cut 

 and some due to quarrying, the following Mesozoic rocks 

 appear in order : — Trias, Lower Lias, Upper Cretaceous. 

 The basalts are found lying upon an eroded surface of 

 Chalk, and occasionally overstep on to the Triassic sand- 

 stones, as they do at Scrabo HUl. A layer of reddened 

 flint gravel constantly intervenes between the basalt and 

 the chalk, representing the material that covered the 

 surface, as a product of subaerial decay, before the eruptive 

 epoch opened. We can picture, then, a country of low 

 chalk downs, the dark beds of the Lias and the red-brown 

 Trias occasionally showing in the valleys. Trees grew 

 in sheltered places, and streams collected the flint 

 nodules in their courses, washing them out of the 

 general soil-cap of the country. In the great period of 

 stress, which gave rise to the Pyrenees and the Juras, and 

 ultimately to the Alpine system, the north of Ireland and 

 the west of Scotland became broken by a series of fissures, 

 up which molten lava flowed. These fissures remain to 

 us as an amazing series of dykes, traversing the area in a 



* •' liecherches sur Us volcaiis eteints dii T'ivaraU et du Velay," 

 Grenoble and Paris, 1776. 



+ See Sir A. Geikie, on Guettard and Desmarest, '■ Ancient 

 Volcanoes of Great Britain " Vol. I., preface. 



J"Eep. on Geol. of Londonderry, etc." (184.'?), pp. 37-44. 



