and the Palaozoic System of England. 473 



alluded to, as a distinct formation — separable from the so-called 

 " Lower Silurian " rocks, and constituting a physical and palse- 

 ontological base to the true " Silurian System :" and lastly, that 

 '' System/^ when reduced to its true base, is, we believe, either 

 in actual position, or in palseontological succession, discordant 

 to the Cambrian rocks on which it rests. 



If these conclusions be true, there is an end of any legitimate 

 dispute on nomenclature; for we have no example in English 

 geology of two great formations which are, as a general rule, 

 unconformable in their position, yet at the same time belong 

 to a common series, and pass under a common name. 



Having thus completed our observations on the groups con- 

 nected with the May Hill sandstone, we next examined the sec- 

 tions through the Llandeilo group in the valley of the Towy. 

 The results of this examination, and their bearing on the questions 

 of palaeozoic classification and nomenclature, will form the con- 

 cluding part of the paper, which I now respectfully offer for 

 insertion in the Philosophical Magazine and Journal. 

 I have the honour to be. Gentlemen, 



Your faithful Servant, | 



A. Sedgwick. 



Cambridge, October 31, 1854. 



I have thought it best to retain the title adopted in my former 

 communication, published in the October and November Num- 

 bers of this Journal. When the substance of this paper was 

 read before the British Association (during September last) at 

 Liverpool, the title was somewhat modified, for the express pur- 

 pose of bringing the communication into a direct comparison 

 with one of the preceding year, read before the British Asso- 

 ciation assembled at Hull, of which an abstract has been pub- 

 lished in their volume for 1853. 



The above letter to the Editors is perhaps h sufficient intro- 

 duction to the new facts about to be described ; but for the sake 

 of any reader who is not familiar with the questions in debate, I 

 think it best to give, in the first place, a short summary of the 

 facts brought forward in my previous paper, and of the conclu- 

 sions drawn from them. My special object was to prove — 

 (1st) that the May Hill sandstone is the true and only base of 

 the Silurian series (or " system ") ; (2ndly) that the Silurian 

 series, when thus defined, was, as a general rule, either obviously 

 unconformable to the Cambrian series, or overlapped its beds in 

 such a manner as absolutely to conceal the true sequence of the 

 deposits, as we pass from one series to the other. In confirma- 

 tion of these conclusions (and in addition to the sections brought 



Phil. Mag. S. 4. Vol. 8. No. 54. Dec. 1854. 2 I 



