1852.] SEDGWICK ON THE LOWER PALAEOZOIC ROCKS. 149 



thickness, made up of arenaceous flags and grits, sometimes of coarse 

 structure. It occupies a trough, on the east side of which the Bala- 

 Umestone is repeated ovei;; again. This arenaceous deposit was the 

 highest member of my original Cambrian series ; and I need not in- 

 form the Society that it is now identified, in the Government Survey, 

 with the Caradoc sandstone. 



Beyond this eastern line of the Bala-limestone, there is an outcrop 

 of older Cambrian rocks* ; after which the whole sequence is broken 

 by enormous faults. The strike of the beds is suddenly shifted ; irre- 

 gular and newer fossiliferous beds appear on the east side of the Ber- 

 wyns in a state of extreme contortion, and with a new strike. But 

 through the range of these contorted beds runs an irregular axis of an 

 older Cambrian group, which throws the shelly masses on one side 

 towards the north, and on the other towards the south. On the 

 north side they are finally carried under the Denbigh flagstone ; to 

 the south, after many undulations, they pass under the flagstone series 

 of Meifod and Welsh Pool. 



That the flagstones of Welsh Pool and Denbighshire were nearly 

 on one parallel, I never had a doubt since 1832. They both be- 

 longed to one series, afterwards called " Upper Silurian." But what 

 were the limestones and shelly sandstones of Meifod and Llansain- 

 fFraid ? I could connect the Llansainffraid beds with the beds at the 

 north end of the Berwyns by an unbroken line of strike ; and there- 

 fore the Llansainff'raid beds (and consequently the Meifod beds) were 

 a part of the Cambrian series ; and the fossils seemed to sanction this 

 conclusion, for the Meifod fossils and Bala fossils seemed to be almost 

 identical in species. 



Such is the great Cambrian series, as determined by myself, after 

 nearly nine months of hard labour, during the summers of 1831 and 

 1832 ; and such was, on all essential points, the account I gave of it 

 before the British Association in 1833, — a great series of deposits, 

 commencing to the east of the Menai, and rolling through the moun- 

 tains in rapid undulations, till the base-line is repeated in the Merio- 

 neth anticlinal. From this base-line to the top of a portion of the 

 Berwyns, the whole series is exhibited in an ascending section, which 

 displays, in order, the four successive groups of the tabular view, 

 collectively not less than 20,000 feet in thickness. 



We now know, through the noble map published under the direc- 

 tion of Sir H. De la Beche, that the highest group of the great 

 ascending section is the equivalent of the Caradoc sandstone of the 

 " Silurian System." Hence this group, as interpreted by the Go- 

 vernment Surveyors, would be common to the Cambrian and Silurian 

 rocks, described by Sir R. I. Murchison and myself, — the highest 

 Cambrian group of my section being coincident with what they re- 

 gard as the true Caradoc sandstone ; and it is this supposed overlap 

 which introduces the only real ambiguity in the development and 

 nomenclature of the lower palaeozoic rocks of North Wales f . 



* Proceed. Geol. Soc. vol.iv. p. 253. 



t To make this more clear, I may state, that the Caradoc sandstone of the well- 

 known Horderley section contains numerous fossils of the Bala group, and none of 



