DISLOCATIONS CAUSED BY TRAP ERUPTIONS. 



461 



from the northern side in Michaelwood Chace, where the greatest volcanic vent seems 

 to have been opened. 



We may further remark that it is scarcely possible to cast the eye over the annexed 

 map without supposing, that the forcible intrusion of the trap must have had a powerful 

 influence, in throwing the sedimentary deposits of this tract into the broken, contorted 

 and complicated forms in which we now find them. Looking at the position of the 

 Silurian rocks of Tortworth, we perceive, that they constitute the termination of a line of 

 elevation proceeding from N.N.W. to S.S.E., and which originating in the valley of 

 Woolhope, passes by May Hill, Aram near Newnham, the Milkmaid rock and Purton 

 passage ; from whence they are prolonged through Michaelwood Chace into the re- 

 markable prongs which embrace the Old Red and Carboniferous deposits. This bi- 

 furcation of the Silurian rocks, has precisely the form which sedimentary deposits 

 would assume, from igneous matter having been emitted along such lines, since 

 besides the chief masses of trap which protrude in irregular bosses, at many points 

 within the area of little more than a square mile, the rock strikes out thence as 

 from a centre, and takes a forked direction. One mass points S.S.E. through Priestly 

 Wood, the other passing to the W.S.W. disappears beneath the surface at Avening 

 Green, but finally reappears at intervals at Charfield Green, in small irregular dykes. 

 The highly inclined, truncated and broken edges of the Silurian rocks along the external 

 edges of the area which they occupy ; and the domes and arches in which they appear 

 near Falfield and Whitfield, all harmonize with the supposition, that the volcanic matter 

 which has reached the surface in the environs of Michaelwood Chace, has not been able 

 to rise from beneath its rocky covering on the south, but in struggling to find a vent 

 has there given rise to the curvatures and breaks of the overlying accumulations 1 . 



It was formerly my belief, that this elevatory influence did not extend beyond Mil- 

 bury Heath ; but having recently visited that part of the district, accompanied by 

 Mr. S. Stutchbury, my attention was called by him to a remarkable tongue of Old 

 Red Sandstone, which extending beyond the limits I had assigned to my map, runs 

 from E.N.E. to W.S.W., through the carboniferous limestone of Old Down. 



This narrow ridge is in truth a distinct prolongation of the axis of Milbury Heath. 

 On one side the beds dip to the S.S.E. beneath the limestone of the Ridgeway and 

 Alveston, whilst on the other the Red Sandstone presents a mural escarpment, against 

 which the ends of the slightly inclined strata of the limestone are abruptly truncated • 

 thus exhibiting a line of powerful fault on the north-western side of the anticlinal. 

 (PL 36. f. 22.) This limestone of Alveston Down is in part of oolitic structure, and con- 

 tains several well-known fossils. Where it is in contact with the Old Red Sandstone, 

 the whole mass for a width of twenty or thirty paces has been powerfully affected, and 



1 Charfield Green may have been elevated en masse, as the country rises on approaching it from east to 

 west. 



3 M 



