194 H. HICKS ON THE SUCCESSION OF 



divisions in the series which were indicated in the present paper. 

 He stated that he traced the line between the blue flags and the 

 Cambrian slates, and believed that an unconformity probably exists 

 between the Tremadoc slates and the Lower Llandeilo. 



Prof. Hughes observed that the fossils by which the rocks under 

 discussion were subdivided did not occur all through the several 

 groups, but only in widely separated zones, and that between those 

 zones sometimes one line and sometimes another had been taken as 

 the arbitrary boundary, often to be shifted in consequence of the 

 discovery of other fossiliferous bands. The line referred to by Prof. 

 Eamsay as that which he was tracing in Worth Wales for the base 

 of his Llandeilo, was a most useful line to draw, as helping to trace 

 horizons, but was not shown to be coincident with any great break 

 in succession. The Silurian system had not, and, after several 

 changes, has not for its' upper boundary a line representing any 

 break in the continuity of deposition. Nor had it at first nor has it 

 now, after much modification, any well-defined natural boundary 

 for its base-line. The only break in it is that which occurs at the 

 base of the May-Hill Sandstone ; and that was unrecognized till 

 pointed out by Prof. Sedgwick many years after the publication of 

 the ' Silurian System,' the author of which, seeing that his system 

 had no base on which to rest, took in group after group of the 

 underlying series, and to justify himself had to prove at each step 

 that as yet no break had been found in the series ; till at length he 

 got down to the lowest Cambrian, part of which he included in his 

 Primordial Silurian. It was now well known, and that chiefly 

 through the labours of Mr. Hicks, that no strong line could be 

 drawn there, and we must therefore take it down to the bottom of 

 the Cambrian conglomerate, or up to the base of the May-Hill 

 Sandstone. Between these horizons lie the Cambrian rocks of 

 Prof. Sedgwick, a well-defined natural group and an ancient name, 

 which, following the true principles of classification and justice in 

 our nomenclature, we must adopt. 



Mr. Ethebidge remarked that several species pass up from the 

 Tremadoc into the Llandeilo, and that the line between the Tre- 

 madoc and the Llandeilo of Sedgwick was not settled. In all cases 

 of this kind stratigraphical or palseontological evidence alone was 

 not sufficient, the two required to be concordant. He entered at 

 some length into the palaeontological statistics of the deposits under 

 discussion, and dwelt especially on the fact that of 70 species of 

 fossils found in the Tremadoc, only 4 pass up into the Arenig. The 

 break at the top of the Tremadoc was thus palaeontologieally of 

 great importance, although not apparent stratigraphically. Hardly 

 any of the Lower Llandeilo (or Arenig) species agree with those 

 of the Llandeilo Elags. The species at the top of the Stiper have 

 a peculiar facies of their own, and would not be recognized as 

 Arenig. 



Prof. Seeley said that the subdivisions of the Cambrian series of 

 Sedgwick were based solely on palseontological evidence, and that 

 to the physical geologist the deposits formed a single series, which 



