Reports and Proceedings — Geological Society of London. 89 

 K-Ei^oiRTS j^nsrn) i^s-ooEiEiDxzsra-S- 



Geological Society of London. 



December 20, 1899.— W. Whitaker, B.A., F.R.S., President, in the 

 Chair. The following communications were read : — 



1. " On some Effects of Earth-movement on the Carboniferous 

 Volcanic Rocks of the Isle of Man." By G. W. Lamplugh, Esq., 

 F.G.S., of H.M. Geological Survey. 

 [Communicated by permission of the Director- General of the Geological Survey.] 



The author, since the completion of his survey of the Isle of Man, 

 has studied the coast-section in the Carboniferous Volcanic Series 

 between Castleton Bay and Poolvash, with the result that he 

 lias discovered evidence that the strata have undergone much 

 deformation in pre-Triassic times, and probably before the Upper 

 Permian rocks of the island wex'e deposited. In the western part 

 of the outcrop the volcanic material consists almost wholly of tuff, 

 in places bedded and fossiliferous ; in the eastern part there exists 

 a chaotic mass of coarse and fine fragmental volcanic material 

 traversed by ridges of basaltic rock and containing entangled patches 

 of dark limestone. The author now considers that the larger 

 lenticles and most of the smaller blocks of limestone have been 

 torn up from the underlying limestone iioor during a sliding forward 

 or overthrusting of the volcanic series upon it. Such blocks do not 

 contain ashy material, and the patches of truly interbedded, ashy 

 limestone can be distinguished from them. The sections at Scarlet 

 Point, Poolvash, and between Cromwell's Walk and Close ny ChoUagh 

 Point are described and figured. Strips of limestoue are found to 

 shoot steeply upwards and become wedged in between the blocks 

 in the agglomerate ; they give indications of sliding and disturbance, 

 and their outer surfaces ai'e indurated and chert-like. The ash, 

 which usually rests on black argillaceous limestone flags, in places 

 appears to come into juxtaposition with a somewhat lower horizon, 

 crushed, platy material intervening between the two. Steep domes 

 of the limestone break into sharp crests which shoot up into the ash 

 and are bent over towards the north ; and between the crests thei"e 

 ai-e small ragged strips of limestone entangled among the tuff. 

 Bands of vesicular basalt are bent and shattered. A dyke-like 

 mass has had a segment sliced off and thrust forward among the 

 agglomerate, so that its flow-lines of vesicles are sharply truncated 

 along the fractured edge, which is often notched and tilled in with 

 ash as though lumps had been torn out of it. The composition of 

 the included masses is influenced by the neighbourhood of the rocks 

 in situ, calcareous near limestone, basaltic near the basalt masses. 



The phenomena may be explained as the result of earth-movement 

 on a group of rocks consisting of limestone passing up into tuff, 

 interbedded with lava-flows, and possibly traversed by sills or 

 dykes of basaltic rock. The effects of tlie disturbance appear to 

 be limited vertically and horizontally, and to have been influenced 

 by the differential resistance of tlie component rocks. Analogous 

 features occur in the Borrovvdale Volcanic Series and in the Silurian 

 volcanic rocks of Portraine. 



