l-i6 PROF. O. T. JONES OX [vol. Ixxvii, 



South Wales is described in some detail. Special reference is 

 made to their development in the neighbourhood of Llandovery, 

 as it was supposed that in that district there was exhibited 



' a passage on the one side into the Upper Sihirian rocks, and on the other 

 into the Upper Cambrian ' (p. 352). 



The fossils in the sandstones in the hillv tract of Xoeth Grug 

 and Cef n-v-o-arreo; were believed to be 



' specifically identical with well-known shells of the Lower Silurian rocks ; 

 while the beds in which they occur gi-aduate on one side into the Cambrian 

 rocks, and on the other into Lower Silurian rocks. Occupying, therefore, 

 the base of the latter system, these beds must underlie the Llandeilo flags, 

 which are here represented by a thick zone of black flag-like beds, though 

 I could find no traces of the characteristic trilobites.' 



These ' Llandeilo Flags ' are represented near Cerig Grwynion 

 on the section accompanying the ' Silurian Svstem ' (pi. xxxiv, 



Murchison remarks that, in the environs of Llandovery, the 

 Lower Silurian rocks are in some places not easily separable from 

 the Tapper by mineral characters. 



While Murchison was attempting to work out the relations of 

 the rocks below the base of the Old Ked Sandstone along the 

 Welsh Borderland and in South Wales, Sedgwick was attacking 

 the far more difficult problem presented by the great series of 

 rocks in North Wales ; he began with the oldest rocks, and worked 

 upwards. In view of the complicated structure of Wales, we can 

 now see how inevitable it was that these two investio-ators would 

 m.eet with rocks of the same age, but of different appearance, in 

 the respective districts with which they dealt. Thus arose the 

 controversy on matters of classification and nomenclature which, 

 however unfortunate it was, nevertheless called special attention 

 to the Welsh rocks, and led to the examination of the problems 

 that they presented by a host of researchers. 



Much of the misunderstandino^ had its orio-in in the usasre of 

 the terms Llandeilo and Caradoc, and in the misinterpreta- 

 tion by Murchison of the structure of that part of South Wales 

 which lies immediately west of the tract called Siluria. He 

 had formed the opinion, without having examined them in detail, 

 that the slaty rocks north-west of Llandeilo, for instance, were, 

 by reason of their more crystalline character, a group older than 

 the shales and mudstones of the Llandeilo and Caradoc. and must, 

 therefore, belong to Sedgwick's Cambrian, Sedgwick, on the 

 other hand, having cursorily examined theu' relations to the rocks 

 of Xorth Wales, regarded them as a development of his Upper 

 Cambrian or Bala Grroup. As this group was later admitted 

 to be in North Wales indistinguishable by its fossil contents fi'om 

 the Caradoc rocks of Murchison, the slaty rocks near Llandeilo 

 should, therefore, be younger than the Llandeilo Series. It is 

 interestins^ to note that the reo:ion which broug;ht the contro- 

 versy to a head was one Avhich neither of the protagonists had 



