1 25 



the time of their deposition.. The older may have been above water, which of 

 course would cause a complete blank in the record, and then after a lengthened 

 period be again depressed, when a new formation would take place over its sur_ 

 face. Thus it is possible that the Lingula flags may not be represented here 

 at all, while elsewheie in Wales they attain some thousands of feet in thick- 

 ness. 



But the relative position of the Stiperstones to the underlying Cambrians 

 is of still further interest, as it has been adduced by Sir C. Lyell to answer the 

 theories of M. De Beaumont, a French geologist whom he speaks of as "a skilful 

 writer and an original observer of great talent and experience." I fear it would 

 be impossible within the limits of this lecture to do justice to either side of the 

 question which has thus been raised. Suffice it to say that M. De Beaumont's 

 theory went to show that " in the history of the earth there have been long 

 periods of comparative repose, during which the deposition of sedementary 

 matter has gone on in regular continuity, and there have been short periods 

 of paroxysmal violence during which that continuity was broken." The evidences 

 of this paroxysmal action he saw in the upheaval of parallel mountain chains, 

 which he inferred, from the position of the strata surrounding them, were up- 

 lifted at the clo^e of each of the quiesctnt periods, and when a new order of 

 animal life was about to be ushered in. This theory Sir Charles has examined 

 with his usual care and impartiality, and has rejected, showing that although the 

 inclination of a particular stratum clearly proves that the mountain chain on 

 which it abuts has been upheaved since its deposition, and so that a fair infer- 

 ence as to the relative age of different chains may be made, yet that it is a gra- 

 tuitous assumption to say that all animal and vegetable life was exterminated 

 when such upheaval took place, or that any interruption even took place in 

 the succession. " A striking illustration of the difficulties we encounter," 

 he says, " when we attempt to apply the theory under consideration even to 

 the best known European countries, is afforded by what is called the ' Sys- 

 tem of the Longmynds.' This small chain, situated in Shropshire, is the third 

 of the typical systems to which M. de Beaumont compares other mountain 

 ranges corresponding in strike and structure. The date assigned to its upheaval 

 is after the unfossiliferous greywacke or Cambrian strata, and before the Silurian. 

 But Sir R. Murchison had shown in 1838, in his Silurian system, and the 

 Government surveyors since that time in their sections (about 1845), that the 

 Longmynds and other chains of similar composition in North Wales are post 

 Silurian. In all of them fossiliferous beds of the lower Silurian or Llandeilo 

 flags are highly inclined and often vertical. In one limited region the Caradoc 

 Sandstone, a member of the Lower Silurian, rests unconformably on the denuded 

 edges of the inferior (or Llandeilo) member of the same group, whilst in some 

 cases both of these sets of strata are upturned." 



It would seem from these remarks that the upheaval of the Longmynds 

 took place after the deposition of the Llandeilo, but before that of the Bala 

 beds, of which they form a part. Now, no violent break can be affirmed to have 



