Aug. 20, 1874] 



NATURE 



325 



Judd in the Island of Mull on the one hand, and by the trachytes 

 of Mont Dore in Central France on the other. They have 

 been described in this district by Berber and Bryce ; but it is 

 only recently that their relations to the other lavas have been 

 clearly determined ; and as such rocks are exceedingly rare in 

 the British Isles, I trust the members of the Association will take 

 this opportunity of paying a visit to the quarries near Antrim, 

 where they are fully opened to view. In composition, both at 

 Hillsborough and at Antrim, they present a felspathic base, 

 enclosing crystals of sanidine (or glassy felspar) and grains of 

 quartz. At Brown Dod Hill they are disposed in sheets, show- 

 ing lines of viscous flow and dipping beneath the overlying beds 

 of basalt. 



As I have already stated, the outpouring of these trachytic 

 lavas may, with every probability, be referred back to the later 

 Eocene period. At any rate, a considerable interval probably 

 elapsed before the eruption of the next series of lavas of Mio- 

 cene age, which are essentially augitic, and may be compre- 

 hended under the heads of basalt and dolerite with their amyg- 

 daloidal varieties. Sheets of these lavas were formed, from 

 various vents, over the uneven surface of the older rocks, and to 

 a far greater extent, both as to area and thickness, than in the 

 case of the preceding eruptions of trachyte.* These beds, 

 Avhich are often vesicular, attain in some places a thickness of 

 600 feet, and are surmounted by decom]iosed lava and volcanic 

 ashes, which mark the close of the second period of eruption. 



The sheets of augitic lava which were poured forth during 

 this stage are remarkable for their vesicular character and the 

 numerous thin bands of red ochre (bole or laterite) which sepa- 

 rate the different lava-flows, and which have been recognised by 

 Sir C. Lyell as probably ancient soils formed by the decomposi- 

 tion of the beds of lava, similar to those in Madeira and the 

 Canary Islands, resulting from streams of sub-aerial origin. 

 Microscopic examination bears out this view ; for a thin slice of 

 one of the more compact beds of bole from the north coast 

 showed that the felspar-prisms retained their form, while the 

 augite and magnetite mgredients had passed into the state of an 

 ochreous paste. 



The vesicular and amygdaloidal character of these older beds 

 of lava shows the probability that they have been poured forth 

 under no greater pressure than that of the atmosphere, and 

 together with the evidence derived from the bands of ochre leads 

 to ihe conclusion that they have been erupted over land-surfaces. 

 Some of the vents of eruption are now visible, either in the form 

 of amorphous masses of trap protruded through the sheets, or of 

 great funnels fdled by bombs, broken pieces of rock, and ashes, 

 such as the rock on which is perched the venerable ruin of 

 Dunluce Castle (the ancient stronghold of the MacDonnells), or 

 the neck erupted through the chalk in the coast-cHRs near 

 Portriish.+ One of these old funnels was found by the late Mr. 

 Du Noyer near this place : it forms a portion of the crest of the 

 ridge overlooking Belfast Lough, to the east of Cave Hill, and 

 is within easy reach of members of the Association. 



The period of the formation of the older sheets appears to 

 have been brought to a close by the discharge of volcanic ashes 

 and the formation of an extensive lake, or series of lakes, over 

 the region extending at least from the shores of Belfast Lough 

 to the northern coast of Antrim, in which the reinarkable beds 

 of pisolitic iron-ore were ultimately deposited. This is the only 

 mode of origin of these ores which seems to me at all probable ; 

 and I am consequently unable to accept the views advanced by 

 Messrs. Tate and Holden regarding tlieir origin from basaltic 

 lava by a process of metamorphism. That water was present, 

 and that thu beds of ash which underlie the pisolitic ore were 

 stratified, at least in some instances, is abundantly evident upon 

 an examination of the sections at Ballypalidy, Ballymena, and 

 the northern coast. In some places they are seen to be perfectly 

 laminated in a manner that could only take place by the agency 

 of water. J It would seem, therefore, that by the combination 

 of slight terrestrial movements, a shallow basin was formed over 

 the area indicated, which received the streams charged with iron 

 in solution, draining the upland margins, from the waters of 



* In this respect they resemble the corresponding rocks in Central 

 France, wh«re, as Mr. Scrope has shown, the trachytes have a more re- 

 stricted ran^e than the basalts (" Volcanoes of Central France "). 



t A sketch of this old rock is given by Prof Geikie in Jukes's " Manual of 

 Geology," 3rd edit. p. 27T. 



X The authors referred to, while admitting the stratified character of the 

 beds at Ballipalidy and their formation in presence of water, consider that in 

 alt other cases the iron ore has been formed on a terrestrial surface: but 

 sections seen at Ballymena and the north coast have led me to conclude that 

 these beds ar« all more or less stratified, and due to aqueous deposition. 



which were precipitated the iron, possibly by the agency of con- 

 fervoid alga;, as in the case of the Swedish lakes of the present 

 day (a view maintained by Mr. D. Forbes, F.R.S.), or by the 

 escape of carbonic acid, owing to which the iron became oxi- 

 dised and was precipitated. 



Upon these uplands grew the plants whose remains occur 

 amongst tlie ash-beds of Ballypalidy, the Causeway, and else- 

 where, and which have enabled Mr. Baily to refer the strata in 

 which they occur to the Miocene period.* In some places the 

 vegetation crept over the surface of the former lake-bottom as it 

 became shallower or Vfas dryin^ up, and gave rise to beds of 

 lignite similar to those described by the Duke of Argyll as oc- 

 curring at intervals amongst the basalts of MuU.i- The beds of 

 ore, wherever they are found, belong to one and the same geo- 

 logical horizon, and enable us to separate the basaltic series into 

 two great divisions — one below and the other above the position 

 of the pisolitic ore ; and which, on maps of the Geological Sur- 

 vey, will for the future be represented by two different shades of 

 colouring. 



The ore itself is now laid open in numerous adits driven into 

 the hill-sides, or in open works at Island Magee, Shane's Hill, 

 Ijroughshane, Red Bay, Portfad, and otiier places,t whence it is 

 transported to the furnaces of Scotland, Cumberland, Lanca- 

 shire, and Wales. A new source of industry and wealth is 

 rapidly springing up over the already prosperous county of 

 Aatrim, and ere many years are over we may expect to see fur- 

 naces established at several points for smelting the ores at the 

 min?s from which they are extracted. 



The period of volcanic inaction just described was brought to 

 a close by fresh eruptions of angitic lavas, which spread in mas- 

 sive sheets over the beds of ore, bole, and even lignite, without 

 materially altering their constitution. Thus on the north coast 

 a band of lignite is interposed between the pisolitic ore below 

 and a massive bed of columnar basalt above, which can be fol- 

 lowed and identified by the size and regularity of its columns for 

 several square miles over that district. That this molten rock 

 has not utterly reduced the lignite to ashes, or even entirely 

 obliterated the iinpressions of the plant- remains, has been doubt- 

 less due to the rapidity with which a hard cnist, of low con- 

 ducting power, consolidates on the outside of a lava-stream, 

 as has been frequently observed on Vesuvius and other active 

 volcanoes. 



Above this peculiarly massive bed were piled fresh sheets of 

 basalt and dolerite to a total depth of at least 400 ft., each flow 

 of lava being consolidated in a somewhat different manner from 

 those above and below it, and probably separted from them by 

 considerable intervals of time, as bands of ochre intervene in 

 most instances between successive beds indicating subaerial soils 

 of decomposed lava. 



The maximum thickness of the basaltic sheets of Antrim has 

 been estimated by Mr. Duffin and myself at 1, 100 ft., to which 

 must be added perhaps 200 ft. for the subordinate trachytic beds, 

 giving a total of 1,300 ft. for the whole volcanic series. This is 

 rather more than originally assigned by Dr. Berger, who places 

 it at <)ooft.,§ but it falls far short of the enormous accumulations 

 of Mull, estimated by Prof. Geikie at from 3,000 to 4,000 ft. ; 

 in neither district, however, have we the data for determining 

 the original thickness of volcanic ejccla, as in both large masses 

 of material have been wasted away by denudation, and not a 

 single volcanic cone or crater remains behind out of all those 

 which, probably in numbers corresponding to those of Central 

 France, were planted over the entire volcanic region. 



The basaltic dykes which traverse not only the geological 

 formations subordinate to the bedded traps, but also the latter 

 themselves, are, in some districts, both remarkable and exceed- 

 ingly numerous. To the south of Belfast Lough we find at 

 Scrabo Hill an outlying mass of bedded dolerite resting on New 

 Red Sandstone, and far beyond the limits of the main masses 

 which rise in a fine escarpment to the north of the Lough. 

 There is every probability tliat Sci;ibo Hill is the site of a dis- 

 tinct focus of eruption ; but it is also remarkable for the dykes 

 of trap, as well as intrusive sheets, which have been squeezed 

 in between the beds of sandstone themselves. Admirable and 

 instructive sections are laid opeir in the freestone-quarries of this 



* Quart. Jour. Geol. Soc, vol. xxv. p. 357, pis. 14 and 15. The plants 

 determined by Mr. Baily, from Ballypalidy, belong to the genera Sequoia^ 

 Cnpressiies, Rlianmus, QiKrciis, Pitius, &c. They were originallydetected 

 by the late Mr. Du Noyer. 



t Jukes's " Manual of Geology," 3rd edit. p. 690. 



t At Pleaskia Head it was originally observed by the Rev. Dr. Hamilton 

 (1790). 



§ Trans. Geol. Soc, lit Series, vol. lii. 



