1846.] SEDGWICK ON THE FOSSIL SLATES OF N. WALES, ETC. 161 



I never attempted to meet the difficulty just stated, till some 

 time after the publication of the " Silurian System." My mind was 

 then employed on the details of the " Devonian System," to which 

 I devoted the larger portion of five summers. But so far back as 

 1833, I explained to the British Association, and afterwards to this 

 Society, a section extending from the shores of the Menai to the 

 edge of Shropshire, and exhibiting a series of symmetrical undula- 

 tions through a great succession of Cambrian slates, forming one 

 great physical system, which I did not then pretend to subdivide 

 into distinct zoological groups, as 1 have done in the present paper. 

 But I did state, that the highest group of this great undulating series 

 (to the east of the Berwyns and in the neighbourhood of Llangollen) 

 was apparently the equivalent of the lower group (afterwards called 

 Caradoc sandstone) in the typical country of Siluria. I also pointed 

 out, that a great physical group (which I have since called Denbigh 

 flagstone) distinctly overlaid all the rocks above mentioned, and that 

 it was identical with a similar flagstone group in the neighbourhood 

 of Welsh Pool, to which the name " Upper Silurian " has since been 

 given. 



It is obvious from this statement, when the words " Silurian Sy- 

 stem " were used to define the whole series of rocks in Siluria, that 

 the lower groups of th^t system overlapped the upper groups of the 

 great North Welsh sections, without including in those sections the 

 still higher groups of " Denbigh flag " (Upper Silurian). Still it 

 appeared to me absolutely certain that the greatest portion of the 

 undulating series of North Wales was inferior to the lowest rocks of 

 Siluria ; and on that ground I, from the first, objected to the word 

 system as applied to the rocks of Siluria, believing them to have, as 

 already stated, no well-defined base, either physically or zoologically. 

 A continuous base-line was however drawn, in the map of the 

 " Silurian System," between the system of rocks in Cambria and 

 the system of rocks in Siluria. With the exception of a demarcation 

 of a few miles in length, at the north end of the Berwyn chain, by 

 which I endeavoured (at the request of Sir R. I. Murchison) to cut 

 off from the highest Cambrian rocks certain fossiliferous beds which 

 I supposed of the age of the Caradoc sandstone, I am in no way 

 responsible for any part of this base-line. It was not I that cut off 

 the older Cambrian rocks from the Silurian ; but Sir R. I. Murchi- 

 son that cut off the Silurian rocks from the Cambrian. This remark 

 is not unimportant, as my share in this demarcation has been mis- 

 represented in one of the published memoirs of our Society. That 

 the demarcation is erroneous is admitted by the author of the Si- 

 lurian system, and he has now expunged his base-line; and in maps 

 subsequently published, has removed it hypothetically to the western 



Orthoceratite, &c. And in like manner, the Secondary system would, in its 

 widest sense (as a first or primary division), contain all rocks with the secondary 

 types of organic life, such as Ammonite, Belemnite, &c. But I am not permitted 

 to follow out a discussion which goes beyond the immediate objects of this com- 

 munication. 



