BASAL CAMBRIAN ROCKS OF SHROPSHIRE. 415 



did not exist when the grit was being formed. This grit must, there- 

 fore, antedate the (juartzite, and the elevation of the hills into a 

 barrier mnst have occurred during the interval between them. And 

 this is consonant with what we find elsewhere. ^Fuch of this grit 

 lies below the conglomerate of the Longmynd, which was derived 

 from the volcanic hills when first they came under the action of the 

 denuding forces. As they rose they supplied the materials for the 

 deposits on the west and formed the boundary of a quiet sea on 

 the north, where organic life began to flourish. The fauna of the 

 quartzite and the Comley sandstone, which everywhere succeeds 

 it, and which is, therefore, probably only its continuation, is believed 

 to be the oldest in the British Isles, except the faujia of the Monian 

 rocks ; but this does not necessitate its development at the earliest 

 portion of the Cambrian period, when we remember how vast an 

 amount of unfossiliferous Cambrian there is in North Wales, and 

 how far above the base the Paradoxides-7.one is at St. Davids. 



2. lli.e SucceecUmj Foruiations. 



This barrier, once established, continued to be an important 

 feature in the district through many subsequent ages. Now it is the 

 character of Cambrian rocks, whenever we find them, to form the 

 base of a continuous series which succeeds in regular order in the 

 same district. On the other hand it is the character of Pre- 

 Cambriau rocks to form the nuclei down to which the denudations 

 of later ages have cleared away the rocks, so that they constantly 

 form limits to later formations. This they do in Anglesey, at 

 Charnwood, and here. For example, the next group above the 

 primordial beds is the Caradoc. Towards the north this is separated 

 from tlie volcanic hills by an expansion of the former ; but it reaches 

 them at the Lawley and Caer Caradoc, wlicrc it is faulted again, 

 then on the other side of Cardington Hill the conglomeratic and 

 gritty base of the Caradoc Series may be traced runnii:ig nearly east 

 and west in bay-like curves as far as Hope Bowdler, and showing 

 large rhyolite pebbles wheix' it crosses the Church-Stretton road. 

 Another conglomerate, probably referable to this series is seen at the 

 south end of llagleth ; coarse Caradoc bounds also all the southern 

 inlier towards the east, but it never crosses tlu' main fault. This 

 fault, then, must lie also near the boundary of the ('aradocian sea, 

 since the beds of the same age in the Shelve district have not the 

 same character. The existence of this boundary is probably con- 

 nected with the Tost-Cambrian squeeze which gave the dip to the 

 purple grits and slates and ])laced them in a position for denudation. 



The next formation which comes up to the nucleus is the Silurian 

 with its basal Llandovery beds. The much wider range of these 

 indicates that a depression had sot in. Nevertheless the nucleus 

 appears to have formed their northern boundary. They commence at 

 Little Stretton and may be traced continuously, clinging to the 

 Longmynd rocks, all the way to Plowden. They are mostly lost 

 .sight of in tlie succeeding vallc}-, but are clearly seen all the way 



