CHAP. XXXVII 
THE ANTRIM PL A TEA U 
205 
volcanic rocks and the washing of their tine detritus by water. Possibly 
this decay may have been in part the result of solfataric action. From 
true bauxite or aluminium-hydrate, the sediments vary in composition and 
specific gravity and pass into aluminous silicates and iron-ores. They 
seem to indicate a prolonged interval of volcanic quiescence when the lavas 
and tuffs already erupted were denuded and decomposed. 1 
The area over which this interesting series of stratified deposits now 
extends is obviously much less than it was originally. It has indeed been 
so reduced by denudation into mere scattered patches that it probably does 
not exceed 170 square miles. But the group can be traced from Divis 
Hill, near Belfast, to Eathlin Island, a distance of 50 miles, and from the 
valley of the Bann to the coast above Glenarm, more than 20 miles. There 
can be little doubt that it was once continuous over all that area, and that it 
probably extended some way further on each side. If the so-called Pliocene 
clays of Lough Neagh be regarded as parts of this group of strata, its extent 
will be still further increased. Hence the original area over which the iron- 
ore and its accompanying tuffs and clays were laid down can hardly have 
been less than 1000 square miles. This extensive tract was evidently the 
site of a lake during the volcanic period, formed by a subsidence of the 
floor of the lower basalts. The salts of iron contained in solution in the 
water, whether derived from the decay of the surrounding lavas or from 
the discharges of chalybeate springs, were precipitated as peroxide in 
pisolitic form, as similar ores are now being formed on lake-bottoms in 
Sweden. For a long interval, quiet sedimentation went on in this lake, 
the only sign of volcanic energy during that time being the dust and stones 
that were thrown out and fell over the water-basin, or were washed into 
it by rains from the cones of the lava-slopes around. 
It may here be remarked that the tendency to subsidence in the Antrim 
plateau seems to have characterized this region since an early part of 
the volcanic period. The lake in which the deposits now described accumu- 
lated was entirely effaced and overspread by the thick group of upper basalts. 
But long after the eruptions had ceased, a renewed sinking of the ground 
gave rise to the sheet of water which now forms Lough Neagh. 2 
Nowhere else among the Tertiary basalt-plateaux of Britain has any 
trace been found of so marked and prolonged a pause in the volcanic 
activity as is indicated by the Antrim zone of tuffs and clays. Through- 
out the Inner Hebrides, indeed, numerous intercalations of sedimentary 
material occur among the basalts, but these consist mainly of tuffs and 
volcanic conglomerates with less frequent shales and coal - seams, and 
they never suggest so distinct and lengthened an interval as is indicated by 
the Antrim deposit. 
It is not improbable that this interval was marked by the outbreak of 
rhyolitic eruptions somewhere in the region. The abundance of rhyolite 
1 See a note 011 Bauxite l>y Professor G. A. Cole, Scicntif. Trans. Royal Dublin Hoc. vol. yi. 
series ii. (1896), p. 105. 
2 This subject will be discussed in Chapter xlix. 
