ON THE CAMBRIAN AND SILURIAN OF THE DEE VALLEY. "Z i i 



18. On the Cambrian (Sedgiu.) and Silurian Beds of the Dee 

 Yallet, as compared with those of the Lake District. By 

 J. E. Marr, B.A., F.G.S. (Read June 2b, 1879.) 



In comparing the beds of the Dee valley * with those of the Lake 

 district, we find a strong similarity in strata referred to the same 

 age, as we might expect, considering the proximity of the two 

 areas. The chief differences appear to be due to the two areas 

 having been affected by intense volcanic action at somewhat dif- 

 ferent periods, the effects of which were felt throughout the ages 

 which followed the period of outburst. This had the effect of 

 causing a local physical break to occur earlier in the northern than 

 in the southern district. But such local breaks as occur in volcanic 

 districts, although more apparent than breaks more widely spread 

 in extent, which are frequently somewhat obscure (owing probably 

 to a uniform action of upheaval, and hence a check of denudation), 

 cannot be of any thing like such value as the latter in assisting 

 classification. 



The end of the period of great volcanic activity in the Lake dis- 

 trict, being soon followed by a period of depression (during the 

 deposition of the Coniston-Limestone series), did not allow of much 

 denudation of the volcanic materials. On the other hand, in the 

 North Welsh area, the period of great vulcanicity was soon suc- 

 ceeded by a period of upheaval (viz. that at the end of the Cam- 

 brian epoch); hence the products of volcanic activity were not 

 covered by much sediment, but suffered great denudation ; and in 

 consequence of this the Silurian beds of the Dee valley are, as a 

 general rule, composed of coarser materials than those of the Lake 

 district, and seem to have b^en, for the most part, deposited in 

 Bhallower water. 



It seems very probable that the Snowdonian area was occupied 

 by dry land soon after the last volcanic outbursts there, and that it 

 remained so during the Silurian epoch, causing some of the animals 

 of the neighbouring coasts to migrate ever and anon from the 

 shallower-water conditions which obtained in the Dee-valley area 

 to the deeper waters of the Lake district, of which a few examples 

 will be presently pointed out. 



The following table gives, in one column, a list of the Cambrian 

 and Silurian beds of the Lake district ; and in a parallel column 

 are given the corresponding beds of the Dee valley. 



* I have received much valuable assistance from my friend A. S. Eeid, Esq., 

 of S. John's College, Cambridge, in examining the Dee-valley area. 



