part 2] THE YALEXTIA.:Nr SERIES. 147 



examined in detail. It was Sedgwick's contention that Murchison 

 had misinterpreted the position of the Llandeilo rocks, while 

 Murchison claimed that the Upper Cambrian of Sedgwick could 

 be proved by its fossils to be equivalent to the Caradoc and 

 Llandeilo Grroups. 



It is now known that in this, as in other cases, Sedgwick's 

 stratigraphical woi*k was wonderfully good, and there is no doubt 

 that, as a group, the Bala was more homogeneous, and rested on 

 a firmer stratigraphical basis than the Llandeilo and Caradoc of 

 Murchison. 



The inferior limit of the Bala was finally taken at the base of 

 the black shales overlying the highest porphyries of Arenig and 

 Cader Idris, while the superior limit was defined by the base 

 of the Wenlock, the lowest of Murchison's groups, the relation of 

 which to the adjacent formations could not be called in question. 



A difficulty arose in parts of North Wales, inasmuch as a group 

 of sandstones there underlay shale of Wenlock type. These the 

 officers of the Geological Survey and Murchison for many years 

 regarded as Caradoc (' Welsh Caradocs '),i because, like those 

 rocks in Shropshire, thej'' formed a sandstone group underneath the 

 Wenlock. Sedgwick proved, by an examination of their fossils, 

 that they were indistinguishable from the Wenlock Shale above 

 them, and he definitely grouped them with that formation under 

 the name of Denbighshire Sandstones, the upper part of the 

 Bala being then formed by pale, earthy, slat}^ rocks upon which 

 those sandstones rested. 



A few years afterwards Sedgwick, with the assistance of 

 McCoy and Salter as palaeontologists, devoted special attention 

 to the fossil contents of the Caradoc of Murchison and the Geo- 

 logical Survey.2 He was able to show that the rocks of the 

 May-Hill district which had been called ' Caradoc Sandstone ' 

 were intimately allied to the overlying Wenlock rocks, and 

 should in reality be regarded as a basal type of Wenlock. He, 

 therefore, proposed ^ for them the name of ' May Hill Sandstones,' 

 which had, in fact, been applied to them by Murchison in one of 

 his early papers. He proved, also, that they were totalh' distinct 

 from the main mass called ' Caradoc ' in Shropshire ; but he failed 

 in the time at his disposal to recognize these May Hill Sandstones 

 between the typical Caradoc and the Wenlock Shale of Shropshire. 



After the reading of Sedgwick's paper to the Geological Society 

 in November 1852, the Geological Surve^'^ dispatched Aveline 



' to re-examine the boundary -line between the Caradoc Sandstone and the Wen- 

 lock shale along the base ol' the Wenlock Edge, where the '"'■ Pentamenis Beds " 

 had been described (" Silurian System ") as forming an intermediate or passage 

 group from the Lower to the Upper Silurian, and to draw the line at the base 

 of those beds, if they should prove distinct from the lower and more typical 

 portion of the " Caradoc." It was, also, necessary to ascertain whether these 



1 E. I. Murchison, ' Siluria ' 5th ed. (1872) p. 103. 



2 Q_ J (j_ g_ ^Ql^iii ^]g52) p, 136. 



3 Ihid, vol. ix (1853) p. 215. 



