326 



NA TURE 



Aug. .-JO, 1874 



hill, which will amply repay a visit. Another district remarkable 

 for such intrusions is that of Ballycastle, where dykes and sheets 

 are seen traversing the carboniferous rocks, as described by Sir 

 R. Griffilh in his admirable report on the geology of that coal- 

 field ;* while the well-known Giants' Causeway is itself a tesTC- 

 lated pavement of columnar basalt, traversing in the form of a 

 dyke the horizontal sheets of older formation. 



The intrusion of the thi usands of dykes of the notth-east of 

 Ireland is unaccompanied by crumplings cr contortions of the 

 strata ; and if it were possible to place the dykes side by side, 

 their aggregate breadth would cover a space several thousand 

 feet in breadth. How, then, has this additional space amongst 

 ftrala of given horizontal dimensions been oblained? Has it 

 been by lateial tension outwards owing to indation by means of 

 elastic gases or vapours, or by a general bulging of the surface 

 consequent on lateral pressure? The former view, I am told by 

 physicists, is untenable ; the latter is one which will pro- 

 bably prove more consonant with modern views of terrestrial 

 dynamics. 



The results of the microscopic examination of a considerable 

 number of specimens of augitic lavas from various parts of the 

 volcanic district are of a generally uniform character. Whether 

 we take specimens from the largely crystalline granular dolerites 

 of Portrush or Fair Head, or the very dense micro-crystalline 

 basalts of Shane's Castle, the structure and composition are found 

 to be nearly uniform. 



The lava is, with very few exceptions, an amorphous or sub- 

 crystalline paste of augite, enclosing long prisms or plates of 

 labi-adorite felspar, crystalline grains of lilano-ferrite, and often 

 of olivine. Chlorite is also sometimes present as a " secondary " 

 mineral. It will be observed that this diagnosis differs essentially 

 from that assigned by Dr. Zirkel as the normal structure of 

 basalt, in which the base is "a glass," and the other rniner-als 

 (the augite, felspar, and olivine) are individually crystallised 

 out.t This, indeed, is the case wrth the carboniferous mela- 

 phyres of the south of Ireland, J and jirobably with all the rocks 

 in which augite is deficient ; but the basalts of Antrim contain 

 augite so largely in excess of the felspar that it has, in neatly 

 every case, formed the base of the rock.g 



The basalt itself is often so rich in iron as to become an im- 

 pure iron-ore. This is owing to the presence of the metal in the 

 form of minute grains of titaniferous iron-ore, which is the prin- 

 cipal cause of the black appearance of the rock and also as one 

 of the components of the augite. 



From the above general review of the volcanic history of 

 Tertiary times in the north of Ireland it will be evident that it 

 presents us with three distinct periods, similar to those which 

 Mr. Tudd has recognised in the succession of events in the Island 

 of Mull ;— 



The earliest, possibly extending as far back as the later Eoctne 

 period, char'acterised by the trachytic lavas. 



The middle, referable to the Miocene period, characterised by 

 vesicular augitic lavas, tuffs, and plant-beds. 



The latest, referable to a still later stage of the Miocene 

 period, characterised by more solid sheets of basalt and numerous 

 vertical dykes. 



These three stages were pi'obably separated from each other 

 by long intervals of repose and the cessation of volcanic action. 

 The succeeding Pliocene period seems to have been characterised 

 by considerable terrestrial movements, resulting in the produc- 

 tion of fractures in the earth's crust, and (as my colleague, Mr. 

 Ilardman, supposes) in the formation of that lorge depression 

 which was filled with waters having a greater area than the 

 Lough Neagh of the present day. Some of the faults which 

 traverse the upper sheets of basalt, antl are therefore of la'er 

 date, have vertical dislocation amoitnting to 500 or 600 ft., as, 

 for instance, that which runs along the valley under Shane's Hill 

 near Larne. Such great fractures must necessarily liave been 

 accompanied by denudation, and it is probable that many of the 

 present physical features had their origin at this (Pliocene) 

 period. The extent to whiclr the original plateau of volcanic 



' " Geological and Mining Survey of tlie Coal Districts of Tyrone and 

 Antrim " (1829). Some of the sheets in this district ni:iy be of older date 

 than the Miocene age. 



t " Untersnchungen iiber d. mikrosk. Zusammensetzung und Structur 

 dcr Basallgcsteinc " (1870). 



1 E, Hull, " On llie Microscopic Structure of the Limciick Carbonifcro-s 

 Melaphyres," Jonrn. Roy. f'.eol. Soc. Ireland, vol. iii. p. ir2(wilb plates). 



§ Mr. Allpon, F.G.S.,sl. :es (Geol. M.ig. 1S73) that he has found the 

 augite individually crystallised out in a specimen from ne.tr the Causeway. 

 Such a case, however, must be exceptional ; but the rule .is stated above 

 certainly holds good. 



rocks has been broken up and carried away within such com- 

 paratively recent times is vaster than is generally suppo5ed. 

 As there is evidence that the sheets of lava to the north of Pelfast 

 Lough were originally connected with those of Scrabo Hill to 

 the south, we must suppose that this arm of the sea and the 

 valley of the Lagan have been excavated since the Miocene 

 period ; while on the north-west the high elevation to which the 

 escarpment of the basalt reaches, leads to the supposition that 

 the basalf'c sheets spread over the ground now occupied by 

 Lough Foyie. Both along the west and along the eastern sea- 

 board the sheets of lava are abruptly truncated, and must have 

 extended far beyond their present bounds ; while many deep 

 valleys, such as those of Glenarm, Cushendall, and Red Bay, 

 have been excavated. 



But the most remarkable result of the denudation, as bearing 

 upon the subject before us, is the complete obliteration of the 

 volcanic cones which we may well suppose studded the plateau. 

 Some of these cones, at least, were contemporaneous with those 

 now standing upon the granitic plateau of Central France, and 

 which are but little altered in elevation since the fires which 

 once burst forth from Ihem became extinct. But since then the 

 north of Ireland has been subjected to vicissitudes from which 

 Cc ral Fiance has been exempted. The surface of the country 

 has been overspread by the great ice-sheet of the earliest stage of 

 the Glacial peiiod, which appears to have stretched across from 

 the Argileshire Highlands, if we are to judge by the direction 

 of the glacial striie at Fair Head.* 



At a later stage the country was submerged beneath the waters 

 of the Inter-glacial sea which deposited the sands and gravels 

 which overlie the Lower lioulder-clay ; and subset[ucnt emer- 

 gences during the stage of the Upper Boulder-clay, together 

 with atmospheric agencies constantly at work, whenever land 

 has been exposed, have moulded the surface into the form we 

 now behold. 



It will thus be seen that the physical geologist, whether a Vul- 

 canist or a Neptunist, has in this region abundant materials on 

 which to concentrate his attention. 



Vokaiiic Energy. — In connection with this subject it may not 

 unnaturally be expected that I should make some allusion to the 

 views of Mr. Robert Mallet on "Volcanic Energy," which lr» 

 has recently unfolded in the " Philosophical Transactiorrs of the 

 Royal Society. "+ My limits, however, forbid more than a 

 cursory glance at this subject. Stated in a few words, volcanic 

 ener-gy, according to Mr-. Mallet, has its origin primarily in the 

 contraction of the earth's crust, due to secular cooling and the 

 tendency of the interior molten matter to fall inwards and thus 

 leave the exterior solid shell unsupported. The lateral jiressure 

 arising therefrom (which, as Mr. Mallet shows, is vastly greater 

 than the vertical weight of the crust) is expended in crushing 

 portions of the solid crust together, along lines of fracture which 

 are supposed to correspond to those of the volcanic cones which 

 are distributed over the earth's surface. Each successive crush 

 produces an earthquake-shock, and is converted into heat suffi- 

 cient to melt the rocks which line the walls of the fissure or lie 

 beneath at high temperatures, and which, in presence of elastic 

 steam and gases, ^are erupteel at intervals both of time and 

 place. 



In the words of the author 'of these views, "The secular 

 cooling of the globe is always going on, though in a very slowly 

 descending ratio. Contraction is therefore constantly providing 

 a store of energy to be expended in crushing parts of tl-.e crust, 

 and through tiiat providing for the volcanic heat. But the 

 crushing itself does not take place with uniformity ; it neces- 

 sarily acts fcr saltuni after accumulated pressure has reached the 

 necessary amount at a given point, where some of the unequally 

 pressed mass gives way, and is succeeded perhaps by a time of 

 repose or by the transfer of the crushing action elsewhere to 

 some weaker point." 



It cannot be denied that Mr. Mallet's theory seems to be con- 

 sistent with marry observed facts connected with volcanic actien. 

 It has for its foundation an incontestable physical hypothesis, 

 the secular cooling of the earth, and it seems to throw con- 

 siderable light upon several observed phenomena of volcanic 

 action — such as the distribution of cones and craters along great 

 lines, the intermittent character of eruptioiis, and the connection 

 of earthquake-shocks with volcanic outbursts. There are some 

 statements in Mr. Mallet's paper which few physical geologists 

 will be inclined to accept, such as the non-existence of trrre 



• A view also held by Mr. James GeiUie and Mr. Campbell of Ishj-. 



