230 GEOLOGICAL SURVEY IN SCOTLAND CHAP, vn 



not been recognised. It was enough, in Sir Henry De 

 la Beche's opinion, if these ancient rocks were dis- 

 tinguished on the maps by some one common colour. 

 But as the work advanced northwards, and the true 

 significance of the labours of Murchison and Sedgwick 

 began to be perceived, it was seen to be eminently 

 desirable to separate at least some of the larger groups. 

 The great break between Lower and Upper Silurian, 

 which Ramsay had detected near Builth, was one of 

 which he early saw the importance. Sedgwick and 

 M'Coy had shown in 1852 that rocks which had been 

 grouped by the Survey with the Caradoc sandstone in 

 the Lower Silurian series contained such an assemblage 

 of fossils as linked them rather with the Upper Silurian. 

 Hence it was that Ramsay, who felt himself responsible 

 for the mapping of Shropshire and the adjacent tracts 

 of Wales, and was anxious that the Survey maps 

 should be made as accurate as possible, deputed W. 

 T. Aveline and J. W. Salter to re-examine that region, 

 The result of the labours of these two members of the 

 staff was to establish beyond any doubt that the break 

 between the Lower and Upper Silurian series of 

 formations in that part of Britain was complete, and 

 that the so-called ' Caradoc ' of Murchison and ' Bala ' 

 of Sedgwick were palseontological equivalents, the one 

 of the other. It was then evident that the boundary- 

 lines thus established, and which were put on the 

 Survey maps, would need to be carried into South 

 Wales, where hitherto no attempt had been made to 

 show any stratigraphical subdivisions in the series of 

 formations below the Wenlock group. And this 

 southward extension became all the more necessary 

 after Aveline had separated out the ' Tarannon shales ' 

 below the Wenlock group, and had shown what a 



