A Ramble in West Shropshire. 353 



accordingly, lie has in the map which accompanies his work, 

 shaded off the colour which indicates the one into that of the 

 other, in that part of Montgomeryshire, where, as he believes, 

 a distinction may be observed between the two. No doubt the 

 fossils found in the two strata, as represented on each side of 

 the Longmynd, are widely different, at least specifically, yet 

 they may not be more so than might be expected in the upper 

 and lower members of a stratum of such enormous thickness 

 as this appears to be. If this be so, we may consider the rocks 

 on this western side of the Stiperstone and Longmynd range, 

 rising up as they do at a high angle towards the east, to be 

 the continuation of a vast arch, of which the other extremity is 

 the Oaradoc and neighbouring hills, the great centre of up- 

 heaval being along the axis of the Longmynd. 



This sketch of the geology of this district would be incom- 

 plete without drawing attention to a much more recent forma- 

 tion, which is very well displayed in some portions of it. 

 This is the Llandovery, elsewhere divided into two members, 

 the upper and the lower, but of these the former alone has left 

 any traces here ; it is in all these cases, however, seen to rest 

 on very much older rocks, unconformably. Near Norbury it 

 reposes directly on the Llandeilo, and it has left a small patch 

 here and there (as will be seen by reference to the geological 

 maps) in the great valley of Llandeilo, which we have been 

 exploring ; but at the southern extremity of the Longmynd, 

 and along a considerable portion of its eastern flank, it rests 

 on the much more ancient Cambrian, bearing a strong re- 

 semblance to a sea-beach. 



These facts suggest, that between the times of the depo- 

 sition of these strata a great change had taken place in the 

 level and position of the underlying rocks, and it has been 

 suggested as probable that the Longmynd, at the time of the 

 formation of the Llandovery, presented the appearance of an 

 island. It is here interesting again to notice the coincidence 

 of a great change in organic life, with these evidences of a 

 very long interval between the two formations. Out of the 

 prodigious number of fossils found both in the Caradoc and 

 the Llandovery rocks, only twelve species are known to be 

 common to both. When we contemplate these strata, resting 

 as they do upon the highly inclined rocks of the Longmynd, the 

 mind is irresistibly carried back to reflect on the history of this 

 region. There was at first a period during which the 26,000 

 feet of the Cambrian was forming ; then another, during 

 which the Lingula flags of the Stiperstone range were depo- 

 sited. Then there was a vast interval, perhaps of subsidence, 

 represented by the Tremadoc slates. Then again fresh de- 

 posits commenced over the entire area, and 3000 feet of Llandeilo 

 VOL. xi. — no. v. A A 



