178 
THE GEOLOGIST. 
The strike of the underlying limestone is nearly due north and 
south, and the dip is to the west, at a generally low angle. In some 
places the tuft" underlies the " black marble," sometimes it overlies 
it : in some pjaces again they are interstratified, while in others they 
form beds of a mixed character. In all, the characteristic fossils are 
found. The black marble is remarkable as containing the only traces 
of coal-plants found on tiie island, several species of ferns, calamites, 
and lepidodeudra having been found in it. This marble admits of a 
high polish, and is extensively quarried for architectural and other 
purposes. Here the palaeozoic series of the Isle of Man terminates. 
From these rocks, evidently belonging to the lower beds of the coal- 
measures, to the clays and gravels of the Pleistocene group, there is 
a great gap, which we have little hope of ever being able to fill ; 
either the intervening deposits have never had an existence here, 
which is the more probable, or else they have been completely de- 
nuded and no trace of them now remains. Of the history of this 
vast period we are not, however, altogether ignorant, several of the 
numerous faults and dykes found in different parts clearly belonging 
to it ; and we are thus, by tracing their effects, able to gather a few 
meagre particulars respecting the nature of operations which would 
otherwise have escaped all notice. 
Crossing these tufaceous beds and parallel to them are trap dykes, 
which in their passage through them and the subjacent limestone 
have greatly dislocated and contorted the strata. The interesting 
question thus arises, what effect, if any, these igneous intrusions 
had upon these beds ? Again, in the north of the peninsula of 
Langness there is an enormous development of greenstone, and the 
peninsula is crossed in all directions by numerous greenstone dykes, 
one of them more than forty feet broad, where it emerges from under 
the schists. The peninsula itself is a mass of Cambrian schists, 
once undoubtedly covered by Devonian and Carboniferous deposits, 
but of which its central and eastern parts are now completely de- 
nuded. What connection was there between these igneous intrusions 
and the elevation of the peninsula, one consequence of which was 
the denudation of these later beds ? But leaving these and many 
other similar speculations respecting the possible or probable eff'ects 
of the numerous faults and dykes which cross many parts of the 
island like network, we come to notice more particularly a great line 
of fault which has undoubtedly played a conspicuous part in the later 
geological history of the island. This great fault extends from Per- 
wick Bay, half a mile south of Port St. Mary, in a north-eastwardly 
direction, through Port St. Mary, Strandhall, and Athol Bridge, and. 
cuts off abruptly all the Devonian and Carboniferous rocks. To the 
north-west of this line are the Cambrian schists, dipping, at an 
angle of varying intensity, to the south. To the south-east of it 
are the lower beds of the Carboniferous limestone, dipping, at a very- 
low angle, to the east. The uplift is consequently to the north-west 
of this line. Its value cannot, with the scanty facts respecting it in 
our possession, be accurately estimated ; but it must have been very 
