166 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jail. 6, 



the Devonian, instead of continuing to be one natural series divided 

 into two groups, as first established in Britain and subsequently ex- 

 tended to other parts of Europe. 



Painful as it is to me to oppose my distinguished associate,— the 

 friend with whom I have so often cooperated, and to whom I dedi- 

 cated the " Silurian System," I am compelled to resist a proposition, 

 the adoption of which would, I conceive, be productive of great dis- 

 service to the progress of geology ; and he must therefore excuse me 

 if I defend the ground of Caractacus, with a pertinacity approaching 

 to that of the old Silurian chief. 



I shall therefore treat this question under the following heads : — 

 the origin of the terms "Silurian" and " Cambrian" — the nature 

 and progress of the classification to which the term" Silurian" re- 

 ferred, whether in Great Britain or the continent — and the effects of 

 the adoption of the proposed change upon geological science. 



After labouring during four years in certain Welsh and English 

 counties, in which I had traced out and defined a succession of fos- 

 siliferous deposits, from the base of the Old red sandstone down to 

 certain older rocks then called " slaty greywacke," I was urged by 

 many geologists to designate such rocks by a distinct name. Ac- 

 cordingly in June 1835* I applied to them the term " Silurian Sy- 

 stem," thereby to distinguish a series of strata several thousand feet 

 thick, and having a general community of fossil characters within the 

 ancient British kingdom of the Silures. These I divided into two 

 groups, the Upper consisting of the Ludlow and Wenlock, the Lower 

 of the Caradoc and Llandeilo formations ; the chief or typical fossils 

 of each, being identified or named by Mr. James Sowerby and my- 

 self, were then enumerated in tables f. To make this matter clear, 

 I here reprint the very woodcut and lettering used twelve years 

 ago. 



At that time the word " Cambrian " was unknown, and it origi- 

 nated in the following manner:}:. In the accompanying diagram of 

 succession, the Upper and Lower Silurian rocks M'ere represented as 

 reposing on unconformable strata which were termed " slaty grey- 



* Phil. Mag. June 1835, vol.vii. p. 50. 



t Even at that early period I guarded against the possible inference that the 

 mineral masses so defined Avere to be viewed as persistent. I showed, for ex- 

 ample, that in some tracts the lower division exhibited great thickness of sand- 

 stone beneath flags and limestone ; in others vice versa. The same was explained 

 at greater length in the work subsequently pubhslied (1839), wherein the evanes- 

 cence of mineral types being indicated, it was maintained, that the fossils alone 

 could truly indicate the age of the strata. This fact is now mentioned, because 

 notwithstanding all my caution, it has recently been stated that flagstone beds 

 with Asaphus Buchii occasionally lie above sandstone to which the word " Cara- 

 doc " may be applied. My classification, I repeat, was never based on such mineral 

 succession ; for whether the lower rocks were flaglike and calcareous, sandy or 

 quartzose, schistose or slaty ; whether the uppermost bed was sandstone and the 

 lowest schist, or the reverse, their age was alone to be determined by their im- 

 bedded fossils. And as to the mineral base, I will cite my own words: — '' Although 

 the calcareous flags of Llandeilo with their accompanying schists are considered to 

 form the base of the Silurian system, their place is sometimes taken, often indeed 

 they are underlaid by sandstones of considerable thickness." (Sil. System, p. 8.) 



X See also Sil. Syst. p. 8. 



