204 
THE TERTIARY VOLCANOES 
BOOK VIII 
posed between the sheets of basalts. It may he looked upon as probably 
furnishing evidence of the lapse of an interval sufficiently extended to 
permit a considerable subrerial decay of the surface of a lava-sheet before 
the outflow of the next lava. But an attentive study of the plateaux dis- 
closes other and even more remarkable indications that the pauses between 
the consecutive basalt-beds were frequently so prolonged as to allow exten- 
sive topographical changes to be made in a district. Nowhere is the long 
duration of some of these intervals more impressively taught than in 
the central zone of sedimentary strata in Antrim. 
This persistent group of tuffs, clays, and iron-ore is generally from 30 
to 40 and sometimes as much as 70 feet thick. From the occurrence of the 
ore in it, it has been explored more diligently in recent years than any 
other group of rocks in the district, and its outcrop is now known over 
most of the plateau. The iron-ore bed varies from less than an inch 
up to 18 inches in thickness, and consists of pisolitic concretions of 
haematite, from the size of a pea to that of a hazel nut, wrapped up in a 
soft ochreous clayey matrix. 1 Where it is absent, its place is sometimes 
taken by an aluminous clay, worked as “ bauxite,” which has yielded stumps 
of trees and numerous leaves and cones. Beneath the iron-ore or its 
representative, lies what is called the “ pavement,” — a ferruginous tuff, 8 to 
10 feet thick, resting on “ lithomarge,” — a lilac or violet mottled aluminous 
earth sometimes full of rounded blocks or bombs of basalt. The well- 
known horizon for fossil plants at Ballypallidy is a red tuff in this zone. 
The section of strata between the two basalt-groups at this locality may 
serve as an illustration of the nature and arrangement of the deposits. 2 
Upper Basalt, compact and often columnar sheets. 
Brown laminated tuff and volcanic days. 
Laminated brown impure earthy lignite, 2 feet 3 inches. 
Brown and red variegated clays, tuffs and sandy layers, with irregular 
seams of coarse conglomerate composed of rounded and subangular 
fragments of rhyolite and basalt, 3 feet 4 inches. 
Brown, red and yellowish laminated tuffs, mudstones, and bole, with occa- 
sional layers of tine conglomerate (rhyolitic and basaltic), pisolitic 
iron-ore band and plant-beds, 8 feet 1 0 inches. 
Lower basalt, amygdaloidal. 
In some of the Ballypallidy tuffs the most frequent lapilli are pieces of 
green and brown glass, which Mr. Watts compares with the pitehstone of 
Sandy Braes, though rarely containing phenocrysts as that rock does. He 
has found also in these strata a smaller proportion of lithoidal rhyolites 
and occasionally fragments of basic rock. 
The pale and coloured clays that occur in this marked sedimentary 
intercalation have doubtless been produced by the decomposition of the 
1 Consult a good essay on the Iron-ore and Basalts of North-east Ireland by Messrs. Tate and 
Holden, Quart. Journ. Geol. Soc. xxvi. (1870), p. 151. In this paper the nature, composition and 
modes of origin of the iron-ore and its associated strata are fully discussed. 
2 A. M ‘Henry, Geol. Mag. (1895), p. 263. 
