174 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Apr. 20, 



of depth, and supposing the same rule that is now approximated to by 

 physicists obtained when the rocks were formed, the lowest beds of 

 the 40,000 feet thickness would not be heated beyond 700°, unless 

 indeed other Lower Silurian strata, now concealed by the Wenlock 

 shale, still increased the pile. 



Various igneous rocks come through in the main fault, or in the 

 minor dislocations that accompany it. They occur near Kington, at 

 Hanter Hill, Old Radnor, and Stanner Rocks ; and near Church- 

 Stretton, at Cardington, Caer Caradoc, and the Lawley, and, still 

 further north, at Dryton Bank, Charlton Hill, Brockwardine, and the 

 Wrekin. These traps are altogether of a different date from those of 

 North Wales, Builth, the Breidden Hills, and the Shelve country. 

 All these are of the close of the Lingula flags, or else belong to the age 

 of the Llandeilo or Bala period. But the traps that come through in 

 the great line of the Church-Stretton fault are of the date of that 

 dislocation, Aaz. at some time after the close of the Silurian period. 

 Near Church-Stretton the Caradoc Sandstone is highly altered by 

 these traps. Near Kington the traps alter Caradoc Sandstone on 

 Old Radnor Hill, and Ludlow rocks near Hanter Hill. The date 

 of the origin of the fault is unknown. It is later than the deposition 

 of the Coal-measures, because they are affected by it. It is also later 

 than the New Red Sandstone, because it runs into that formation. 

 It is probably of various dates, and its present amount may be the 

 result of various throws. The Caradoc Sandstone east of Caer Ca- 

 radoc is altered by the trap. The Wenlock shale which is brought 

 against the west side of Caer Caradoc by the fault is unaltered. The 

 downthrow has been increased probably on that side since the melted 

 matter was first injected into the crack. In the Permian rocks, in 

 the neighbourhood of the Forest of Wyre, part of Staffordshire, and 

 round the Abberlej^s, there is a trappean breccia, many of the frag- 

 ments of which seem more likely to have been derived from the traps 

 about Church-Stretton, &c. than from any other known rocks. If the 

 conjecture tiiat they have been derived from the waste of these rocks 

 be true, then not only have the traps been injected into a dislocation 

 of older date than the Permian aera, but also denudation has gone on 

 to such an extent, that these originally deep-seated masses have been 

 then exposed at the surface while Permian strata were being deposited. 

 But the fault passes into the Bunter or Upper New Red Sandstone, 

 and this may be on account of a repetition of throw in the old line 

 of dislocation. 



There is one other point to notice with regard to the Longmynd 

 country: — in April 1848 Mr. Aveline and myself described in the 

 Journal of the Society two bands that underlie the Wenlock shale 

 where it bounds the Longmynds on the south-east and south. The 

 uppermost is a band of limestone, the lower is a band of conglomerate, 

 then called Caradoc Sandstone, and which is there correctly said to 

 rest " unconformably on the Longmynd Cambrians, in such a manner 

 that it is plain the latter formed an original boundary of the sea of 

 the period." I may now state that we are agreed to modify that 

 part of the opinion there given which unequivocally refers this thin 



