1846.] kSedgwick on the slate rocks of Cumberland, etc. 129 



in the Cumbrian mountains, it would be among the flagstones asso- 

 ciated with or overlying the Coniston limestone*. 



Though there are several remarkable species of mollusca and 

 Crustacea common to the Wenlock, Caradoc and Llandeilo series, 

 there are other beds far below them which, I believe, contain none 

 of these common species. These beds are subordinate to the most 

 remarkable physical group of England. I have in former papers 

 called it the Protozoic group ; or the lowest and greatest division of 

 the rocks with Lower Silurian fossils. Now that I have no evidence 

 of the existence o^ Asaphus Buchii, and other Llandeilo characteris- 

 tic fossils in this vast group, I am no longer embarrassed for its name. 

 I cannot speak of a Cambrian system, with peculiar fossils found in no 

 other ; but I may speak of the lower or great Cambrian group ; and 

 it is, I think, on very probable evidence, placed on the same level with 

 the green slates and porphyries of Cumberland, which I once called 

 the great Cumbrian group. In this great Cambrian group began 

 the lowest fossil species we know in the British Isles. Many of the 

 lowest species lasted throughout the whole Lower Silurian period ; 

 but new species were added, as conditions gradually changed, during 

 the epochs marked in the ascending sections ; so that the lower fauna 

 reached its maximum of development in the Caradoc sandstone and 

 Llandeilo flagstone. Afterwards the fauna underwent a much more 

 rapid change, certain tribes oi JBrachiopoda diminishing in numbers, 

 and being replaced by other forms, while, as far as our evidence goes 

 (at least in the north of England), the Lamellibranchiata, though be- 

 ginning low in the Cambrian group, also formed a more important 

 part of the fauna of the Upper Silurian rocks. Geology tells us of 

 the successive revolutions and changes in the crust of the earth. 

 Organic changes are our surest guides in making out this history; but 

 they form only a part of our evidence, and the great physical groups 

 of deposits, however rude and mechanical, are historical monuments 

 of perhaps equal importance in obtaining any true and intelligible 

 history of the past ages of the earth ; and after we have descended 

 through a certain number of stages, they become indeed our only 

 monuments and indexes of past events. This is true in North Wales, 



* Of the Coniston Lower Silurian fossils (including Helmsgill, &c.) we find in 

 North Wales — 



Peculiar to the Bala limestone and heds below it 6 species. 



Common to the Bala series and the passage beds 16 „ 



Peculiar to the passage beds 17 „ 



Peculiar to Westmoreland 21 „ 



60 



By * passage beds ' are meant the highest beds of the Caradoc sandstone (e. g. 

 those of Glen Ceiriog, &c.), which form a passage into the Upper system. 



Of the 21 species which are peculiar to this scries in the nortli of England, 

 two occur in the beds on top of the Bcrwyns (Hliiwargor); two in Ireland, in 

 beds referable to Wenlock limestone and shale ; and three are characteristic Wen- 

 lock fossils, namely Cyathophyllum ccespilosum, Cornidiles serpulariusj Tentacu- 

 lites ornatus. 



Of course this list is not absolute ; and the probability is, I think, that there 

 will be no species " peculiar to the Bala series" when we know more. — J. W. S. 



VOL. II. part I. K 



