18 PALEOZOIC TIME. [CHAP. II. 



Archaean rock. But the eastern or Caradoc Archaean ridge 

 is flanked on the other side by a quartzite conglomerate, 

 which is succeeded by sandstones of Middle or Upper 

 Lingula Flag age ; the Lower Cambrian being entirely 

 absent. How can this sudden disappearance of so thick 

 a mass of rock be accounted for ? There seem to be only 

 three possible ways of meeting the difficulty : 



(1) That the Longmyndian sediments were raised into 

 a block of land about the epoch of the Menevian Beds, and 

 that this block afterwards sank again to receive the upper 

 half of the Lingula Flags and the Tremadoc Slates. 



(2) That the Caer Caradoc range was the border of a 

 mass of Archaean land, and formed a precipice or steep de- 

 clivity about 10,000 feet high, so that it was not over-topped 

 by the Cambrian Sea till the time of the Lingula Flags. 



(3) That the faults on the east side of the Longmynd 

 and Caradoc ranges are thrust planes, bringing together 

 portions of districts which were originally many miles 

 apart. But the faults appear to be of the normal kind, 

 and there is no evidence of the great crushing which must 

 accompany lateral thrust. 



If none of these suggestions are considered satisfactory, 

 it can only be argued that the Cambrian age of the Long- 

 mynd rocks is an assumption, and that the facts of the 

 case show them to be physically more closely connected 

 with the Archaean than with the Upper Cambrian. It is 

 indeed quite possible that they are pre-Cambrian, and 

 belong to the interval which is elsewhere represented by 

 the gap between the Archaean and the Cambrian. 



If, however, they should prove to be of Lower Cambrian 

 age, we must select the first of the above hypotheses as the 

 least unlikely, and must suppose that an upheaval took 

 place in the middle of the Cambrian period, but that the 

 land so formed was soon submerged again beneath the 

 sea of the Lingula Flags. 



