160 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DeC. 16, 



types within the geographical limits of the country described in the 

 " Silurian System." 



In our advancing science the nomenclature must undergo changes 

 commensurate to any necessary change of classification or distribu- 

 tion of the great physical groups, otherwise it must inevitably lead us 

 into verbal incongruity, and sometimes into the perpetuation of error. 

 The " Silurian System " never had any good natural base, either 

 physical or geological. Its base-line was in many places drawn arbi- 

 trarily, and in some places, as is now known, erroneously. This 

 assuredly implied that the nomenclature of some of the lower groups 

 in Wales might undergo a partial change to accommodate them to 

 our advancing knowledge. The least change possible, to meet the 

 exigency of the case, would be the best, and above all such a change 

 as did not destroy the original and geographical scheme of nomen- 

 clature, or interfere with the natural development of the successive 

 groups, considered both physically and zoologically. 



Bearing these principles in mind, I adhere to the nomenclature 

 of our older rocks (Cambrian and Silurian, &c.) as it was originally 

 set forth, only because I am, with a very limited exception, con- 

 firmed in my original views respecting the structure of North Wales 

 and the Cumbrian mountains, and the relations of the collective 

 groups one to another. My general views respecting the structure 

 of North and South Wales, since my last revision, are exactly what 

 they were in 1832, however much I may have since then improved 

 my knowledge of subordinate details. In that great country (in- 

 cluding in it also the older rocks of the frontier counties of England) 

 there are, at the least, two great physical systems and two nearly 

 coordinate palaeontological systems. On this point there is no dis- 

 pute which does not admit of very easy adjustment. One is the 

 system of Cambria, the other is the system of Siluria. The only- 

 difficulty is to draw any well-defined line between them*. 



* Should it be contended that the fossil groups of all the rocks here described 

 become so completely blended as to form only one system ; in that case I should 

 not object to a modification of nomenclature, and describe as the collective Cam- 

 brian group what I have called the Cambrian system. This indeed is the very 

 nomenclature I proposed in my former paper (see Journal, vol. ii. p. 129). From 

 the first I objected to the word system (as applied to Siluria) because it was too 

 definite ; that system having no good base-line, either pliysically or zoologically. 

 In the language of Professor Dumont and other continental geologists, instead of 

 Silurian system we should have had " Terrain Silurien ;'' and the word system 

 would liave been applied to its several subdivisions — Caradoc, Wenlock, and Lud- 

 low ; and I think this language would have been more correct than that generally 

 adopted in this country since the publication of the " Silurian System," and that 

 it would not have led to any subsequent misapprehension. Were this the proper 

 place for the discussion, I might state (what I have before stated to the Society 

 during its meetings), that perhaps the best nomenclature of our older rocks would 

 give the name System to the whole Palaeozoic series. In that case the words 

 Cambrian, Silurian, Devonian, Carboniferous, &c. would define subordinate divi- 

 sions or collective groups. These collective groups appear to run together through 

 the intervention of the groups of transition or passage, such as the ''Cambro- 

 Silurian." On this scheme the Palceozoic system would include all the rocks 

 containing the older types of organic Hfe, such as Producta, Orthis, Trilobite, 



