Upper Silurian Rocks. 89 



westward to the mountains of the Longmynd beyond Le 

 Botwood and the beautiful valley of Church Stretton. 



Notwithstanding the unconformity mentioned above, 

 there are in Wales, and partly in Shropshire, rocks 

 containing suites of fossils, many of them peculiar to the 

 horizon in which they occur, and a few common to 

 Upper and Lower Silurian. Part of these strata, such 

 as the Lower Llandovery beds, have been formed during 

 minor oscillatory movements of sea and land. In 

 South Wales, Stricklandinia (Pentamerus) lens occurs 

 plentifully in these Lower Llandovery rocks, and 

 sparingly in the Upper Llandovery rocks ; while P. 

 oblongus occurs sparingly in the Lower Llandovery 

 rocks, and in great numbers in the Upper Llandovery 

 beds, on which rests the Tarannon shale. By far the 

 larger part of the fossils of the Lower Llandovery rocks 

 are, however, essentially of Lower Silurian type, and, 

 besides, they are quite conformable with, and pass by 

 easy lithological gradation into the underlying strata. 



With the Upper Llandovery or Pentamerus beds, 1 as 

 they were formerly called, the case is very different, for 

 in Shropshire they rest unconformably on the Cambrian 

 and Lower Silurian rocks indiscriminately, and possess 

 a beach-like character, being in places formed of pebbles 

 derived from the rocks on which they rest, as in fig. 20, 

 and in Eadnorshire near Built h, the Upper Silurian 

 rocks including these Pentamerus-bearing strata, lie 

 with extreme unconformity, alike on the lowest and the 

 highest Llandeilo and Caradoc beds of that old volcanic 

 area, as shown in figs. 20, 21, 22. 



My belief, therefore, is that these Upper Llandovery 

 beds, which form the true base of our Upper Silurian 



1 See fig. 23, p. 94, Pentamerus oblongus is so common in these 

 strata that they were originally called Pentamerus beds. 



