part 2] THE VALENTIAX SERIES. 149 



examination of the fossils. Aveline further observed that similar 

 pale slates adjoined the Wenlock in many parts of North Wales^ 

 while north of Builth they were proved to underlie the sandstones 

 which the surve3''ors had been in the habit of calling the ' Welsh 

 Caradocs.' The fossils of those sandstones were, tiierefore, care- 

 fully re-examined, and it was shown that they should be assigned 

 to the Wenlock, as had been demonstrated nearly ten years, 

 previously by Sedgwick in North Wales. 



Towards the end of the same year Aveline discovered that * the 

 Pentamerus Sandstones' of Castell Craio- Grwvddon, which had 

 been identified with the Shropshire Peutamerus Beds, were under- 

 lain b}^ another great series of sandstones also containing Penta- 

 menis, the species of whieli were, however, shown by Salter to be 

 different from those of the upper sandstones. The terms ' Upper '' 

 and 'Lower Pentamerus Sandstones' then became current for a 

 short period, until it was decided to give them a more suitable 

 name. Hitherto the upper sandstones had been called by the 

 G-eological Survey, Upper Caradoc, although Sedgwick had pre- 

 viously shown that t\\ey were equivalent to the May-Hill Sand- 

 stones ; while the lower sandstones had received no local name. 

 When the term Upper Caradoc had been shown to be unsuitable, 

 Murchison proposed the name of Llandovery Sandstones to 

 include the lower and upper series (see below). 



Aveline believed at one time that he could separate the Lower 

 Pentamerus Sandstones from the underhdng contorted slaty rocks,. 

 and attempted to map a line at their base. He was forced to abandon 

 this attempt ; but he was able to show that the upper sandstones, 

 were distinguished from the lower, and completely overlapped 

 them in the course of a few miles. The relations of the two 

 groups were, in fact, suggestive of an unconformity between them;, 

 and on this ground it was decided to extend the Upper Silurian 

 down so as to include the Upper Pentamerus Sandstones, and to 

 relegate the Lower Pentamerus Sandstones to the Lower Silurian. 

 This course had previously been proposed by Sedgwick after an 

 examination of the May-Hill district, where, however, the lower 

 group is not represented. In the 4th edition of ' Siluria,' published 

 in 1867, the name 'Llandoverj^ rocks' was introduced by Murchison, 

 and the two groups of sandstones were stjded Lower and Upper 

 Llandovery (p. 85). There still remained as a nameless group 

 the pale shales which intervene between the Upper Llandovery and 

 the base of the Wenlock. Aveline, having traced, in 1855 and 

 1856, shales of this character from Llandovery into North Wales, 

 proposed for them the name of Tarannon Shales, by reason of 

 their great development in that locality. Since that time (1857) 

 until recent years the classification adopted on the maps and in 

 the Memoirs of the Geological Survey was (<?) Lower Llandovery, 

 forming the upper part of the Lower Silurian; ih) Upper 

 Llandovery ; and (c) Tarannon, forming the lower members of 

 the Upper Silurian. The affinities of the Tarannon Shales have 

 given rise to differences of opinion. Ramsay and Aveline were 



