and the Palaeozoic System of England, 489 



If this be true, the ideal section {supra, fig. 5, p. 313) appealed 

 to for years before and after the publication of the ' Silurian 

 System/ as the basis of nomenclature for its lower groups, was 

 absolutely without meaning. If the fundamental sections were 

 wrong, the nomenclature could not be right. In the determina- 

 tion of the true Silurian groups, the author of the " System '' 

 did incomparable service to the cause of geology, and there is 

 not a geologist in the old or new world who is not grateful for it. 

 But in descending below the groups he had irrefragably esta- 

 blished, he missed the key of his own position, and linked toge- 

 ther groups which had no nearer relation than that of accidental, 

 and often discordant, juxtaposition. When he passed below the 

 horizon of the May Hill sandstone, he did the collector's work 

 alii<! the naturahst's work admirably well ; but he failed in the ' 

 real task-work of the geologist — in the definition of the true 

 place of the Llandeilo group in the great Cambrian series ; and 

 without such a definition of the group, its nomenclature, as part 

 of a system, is absolutely untenable and without meaning. And 

 the same conclusion (since the elimination of the May Hill group) 

 applies also to the Caradoc sandstone. 



That the Llandeilo flag is a subordinate part, and not a high 

 part, of the Upper Cambrian or Bala group is, I think, section- 

 ally evident ; but we cannot, by the help of the South-Welsh 

 sections, eliminate its exact place, or its clear coordination with 

 any ascertained stage of the Upper Cambrian group of North 

 Wales, nor is it at all necessary that we should do so. The 

 Llandeilo fossils are well known, especially from the excellent 



which overlie the Cambrian rocks. That the Wenlock shale is in such 

 cases unconformable to the Cambrian rocks is evident from the following 

 facts, which would prove the point independently of the evidence supplied 

 by the above section. To the south of Llandovery this shale ranges over 

 rocks which are geologically higher than the Llandeilo flag; then for 

 about a dozen miles it appears to range immediately over the Llandeilo 

 flag ; and afterwards, in its western prolongation, it again wraps over a 

 group which is, I think, higher than that of Llandeilo. 



It is not always an easy matter to determine whether two contiguous 

 groups be conformable: and if (as in the case before us) the groups have been 

 elevated and contorted by the disturbing forces of a newer epoch, it may 

 become very difficult to make out the fact of discordancy between the 

 groups. I believe that the preceding remark partly explains the great dif- 

 ficulty in making out the true relations of the Upper Cambrian rocks of 

 South Wales. They were first exposed to those disturbing forces which 

 produced the great north-eastern and south-western undulations of North 

 Wales ; and tliey were afterwards crushed, dislocated, and contorted by 

 those great east and west disturbing forces which produced the actual con- 

 figuration of the great South-Welsh coalfield. There can, I think, be no 

 doubt that we owe the principal pheenomena in the valley of the Towy to 

 the action of the latter set of forces upon beds which had been dislocated 

 at a prior period. 



Phil Mag. S. 4. No. 55. Suppl Vol. 8. 2 K 



