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 aiigle. In some 

 places the tuff underlies the " black marble," sometimes it overlies 

 it : in some places 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 the island, several species of ferns, calamites, 

 and lepidodendra 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. 

 Trom 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 

 othervrise 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 effects 

 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 

 g(>()logical history of the island. This great fault extends from Per- 

 wiek J3ay, half a mile south of Port St. Mary, in a north-eastwardlv 

 direction, ihrough Port St. Mary, Strandhall," and Athol Bridge, and 

 cuts olf abruptly all the Devonian and Carboniferous rocks. To the 

 iiorlh-wcst of this line are the Cambrian schists, dipping, at an 

 angle of varying intensity, to the south. To the south-east of it 

 are tlie 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 i)ossession, be accurately estimated ; but it must have been very 



