94 Reports and Proceedings. 



logical Survey have invariably included the Arenig in the Llandeilo 

 group ; but it was now shown that this occurred entirely from a 

 mistaken idea as to the relative position of the two series, which 

 were now shown to be entirely distinct groups,, the equivalents of 

 both groups being present in Carnarvonshire, Shropshire, and Pem- 

 brokeshire, but the Llandeilo group only of the two being developed 

 in Carmarthenshire. 



The lines of division in the series were said to be strongest at the 

 top of the Menevian group and at the top of the Tremadoc group, 

 these lines being pala?ontological breaks only? and not the result of 

 unconformities in the strata. 



Discussion. — Professor Ramsay complimented the author on having brought 

 forward a paper so well worked out. He gave an account of his own early 

 geological work in Wales, and stated that he had mapped the rocks referred to by 

 Mr. Hicks in 1841. He differed from the author in believing his supposed 

 Laurentian rocks to be igneous. They were metamorphosed Cambrian deposits, 

 which had lost all traces of their aqueous origin. In 1-841, no fossils had been 

 found below the Llandeilo Flags in any part of the series described by Mr. Hicks, 

 and thus there was no ground for establishing those palaeontological 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 it would 

 show an unconformity 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 conse- 

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

 Ramsay, as that which he was tracing in North 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. Etheridge remarked that several species pass up from the Tremadoc into 

 the Llandeilo, and that the line between the Tremadoc and the Llandeilo of 

 Sedgwick was not settled. In all cases of this kind stratigraphical or palaeonto- 

 logical 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 four pass up into the Arenig. The break at the top of the 

 Tremadoc was thus palaeontologically of great importance, although not apparent 

 stratigraphically. Hardly any of the Lower Llandeilo (or Arenig) species agree 

 with those of the Llandeilo Flags. 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 palaeontological evidence, and that to the physical geologist the 



