73 REPORT — 1875. 



Llandovery. He criticised the palseontological and other evidence upon which 

 this division had been made, and protested against the introduction of the name 

 Llandovery Rocks instead of May-Hill Sandstone, under which it was first de- 

 scribed by Prof. Sedgwick. In the Lake-district and in North Wales in every 

 open section there was an apparent conformity, though the overlap of the Graptolitic 

 mudstone, from the Coniston Limestone of Windermere to the Ash-Gill Flao-s of 

 Coniston, seemed to suggest an unconformity. The May-Hill Sandstone, thinning 

 out to the north and creeping over the edges of the Cambrian rocks and along the 

 ancient mountain-range of the Malvern and Longniynd, rests on the oldest parts of 

 the Cambrian and even on the pre-Cambrian. Still this cannot be said to repre- 

 sent the previous denudation of the whole thickness of the Cambrian rocks, as they 

 themselves thin out against the old Malvern ridge ; so that this epoch would appear 

 to have been characterized in the typical regions by the upheaval of some mountain- 

 chains and irregular movements in large adjoining areas. 



Wi Epoch. Silurian. — This series he thought commenced with the base of the 

 May-Hill Sandstone {i. e. at the bottom of the Lower Llandovery, with some cor- 

 rections of boundary). There was a very considerable change in the forms of life, 

 and this was conspicuous even where the stratigraphical discordance was not well 

 marked. There was little difference of opinion as to the grouping of beds, except 

 at the commencement and close of the period. Conglomerates mark the base at 

 Austwick and Sedbergh, on the western borders of the Lake country, accompanied 

 by a change in the character and colour of the sediment and of the organic remains. 

 The boundary can be traced through the Lake-district proper, and in North Wales 

 by the same change in the fossils and the sediment, but there is no conglomerate. 

 In South Wales a conglomerate frequently marks the base ; but the group of fossils 

 that comes on first is very different, and seems to suggest an earlier submergence 

 of the southern area. 



Passing over the Wenlock and Ludlow, the next difficulty is in drawing the 

 upper boundary. This he would take at the top of the red shales and marls of the 

 river Sawdde and the country east of Horeb Chapel in South Wales ; for there is no 

 evidence of a break there or anywhere else between the tilestones and the red 

 shales ; and where fossils have been found, as at Ijcdbury, in the red shales they 

 are common Ludlow forms. 



The author pointed out, by reference to original and published sections by Prof. 

 Sedgwick, that the views he now advocated as to the classification of the Cam- 

 brian rocks and the position of the boundary-line between them and the Silurian 

 were exactly those of Prof. Sedgwick. He further showed, by comparison of the 

 map and sections of Murchison with those of the Survey and later authors, that 

 Murchison had not, in 1839, correctly placed any one of the beds about which he 

 later came into collision with Sedgwick ; that the Caradoc of Murchison's sections, 

 supposed to rest on the Llandeilo Flags south of Llandeilo, was May-Hill Sandstone 

 or Wenlock ; that the Cambrian rocks, supposed by Murchison to crop out from below 

 the Llandeilo Flags, were Caradoc and newer beds overlying it ; that the supposed 

 base of the Llandeilo Flags was in fact the top. He further stated that when these 

 errors were corrected there was no acknowledgment of the approach made in the 

 new editions to the original classification of Sedgwick ; that the latest change had 

 carried the base of the Silurian below the unconformities in the Cambrian roclcs 

 given in vol. iii. of the 'Memoirs of the Geological Survey,' and had left it where 

 he thought no one would now venture to suggest there was any palseontological 

 or stratigraphical break. As this must be chano;ed, and the unconformities above 

 mentioned .would, he thought, be certainly abolished before long, he asked whether 

 for justice and consistency we should not, in adopting Prof. Sedgwick's classifica- 

 tion, adopt his nomenclature also. 



loth Ejjoch. The Gap bchveen the Silurian and Carboniferous. — This he con- 

 sidered one of the two most strongly marked gaps (except, possibly, some pre- 

 Cambrian intervals) in all the geologic series. In the north of England the 

 Cambrian and Silurian rocks were folded and denuded down to the Skiddaw 

 Slates : strata to the thickness of at least five or six miles were removed. In 

 the north-west of AVales a similar denudation seems to have been going on ; but 

 as we turn to the east we find, along the Vale of Clwyd for instance, that there 



