~ 
TRANSACTIONS OF THE SECTIONS. 71 
north coast a band of lignite is interposed between the pisolitic ore below and a 
massive bed of columnar basalt above, which can be followed 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 impressions of the plant-remains, has been doubtless due to the 
rapidity with which a hard crust, of low conducting-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 feet, each flow of lava being consolidated in a 
somewhat different manner from those above and below it, and probably separated 
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 1100 feet, to which must be added perhaps 200 feet for 
the subordinate trachytic beds, giving a total of 1300 feet for the whole volcanic: 
series,. This is rather more than originally assigned by Dr. Berger, who places it 
at 900 feet *, but it falls far short of the enormous accumulations of Mull, estimated. 
by Professor Geikie at from 3000 to 4000 feet ; in neither district, however, have we 
the data for determining the original thickness of volcanic ejecta, as in both large’ 
masses of material have been wasted away by denudation, and nota single volcanic 
cone or crater remains behind out of all those which, probably in numbers corre-: 
sponding 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 exceedingly numerous. To the south of Belfast Lough we find at 
Serabo 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 that Scrabo Hill is the site of 
a distinct 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 open in the freestone- 
quarries of this 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. Griffith in his admirable Report on 
the geology of that coal-field+; while the well-known Giants’ Causeway 1s itself 
a tessellated pavement of columnar basalt, traversing in the form of a dyke the 
horizontal sheets of older formation. 
- The intrusion of the thousands of dykes of the north-east of Ireland is unaccom- 
panied by crumplings or 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 strata of 
given horizontal dimensions been obtained? Has it been by lateral tension out- 
wards owing to inflation 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 probably prove more 
consonant with modern views of terrestrial dynamics. 
The results of the microscopic examination of a considerable number of speci- 
mens of augitic lavas from various parts of the volcanic district are of a generally 
uniform character. Whether we take specimens from the largely crystalline gra- 
‘nular dolerites of Portrush or Fair Head, or the very dense micro-crystalline 
basalts of Shane’s Castle, the structure and composition is found to be nearly 
uniform. ge 
The lava is, with very few exceptions, an amorphous or subcrystalline paste of 
augite, enclosing long prisms or plates of labradorite felspar, crystalline grains of 
titano-ferrite, and often of olivine. Chlorite is also sometimes present as a 
“secondary ” mineral, It will be observed that this diagnosis differs essentially 
* Trans. Geol. Soc. Ist series, vol. iii. 
t ‘Geological and Mining Survey of the Coal-districts of Tyrone and Antrim * (1829), 
_ Some of the sheets in this district may be of older date than the Miocene age. 
