154 STKATIGEAPHICAL GEOLOGY 



Kecent researches have shown that the Upper Llandovery is 

 usually succeeded by a series of pale grey and green shales 

 (Tarannon shales), and that these three subdivisions (Lower 

 Llandovery, Upper Llandovery, and Tarannon shales) " are far 

 more closely allied to each other than they are to the beds above 

 or below, and that they should be considered as the three con- 

 secutive members of a single formation." l The Llandoveries were 

 formed in fact during a period of local upheaval and oscillation 

 which intervened between the quiet periods of the Bala and 

 Wenlock Beds. For their equivalents in South Scotland Professor 

 Lapworth has proposed the name Valcntian, from the Roman name 

 of that province, and this is a convenient designation for the 

 lowermost group or series of the Silurian rocks in Britain. 



The second natural division of the Silurian System is typically 

 developed at Wenlock in Shropshire and at Woolhope in Hereford- 

 shire. This series was described by Murchison under the name of 

 the " Wenlock formation," and at the localities above-mentioned it 

 consists mainly of shales with a band. of limestone at the base and 

 another at the top ; but in North Wales and other parts of Britain 

 there are no such limestones, and the only means of delimitation is 

 by the graptolite fauna. 



The third division is the " Ludlow formation " of Murchison, 

 which in Shropshire and Herefordshire consists of shales and mud- 

 stones with a band of limestone in the middle. Although the 

 shales below the Aymestry limestone were placed in this group by 

 Murchison, he admitted that they were merely a continuation of 

 the Wenlock Series, and for some time it seemed doubtful if they 

 could be separated from it when the Wenlock limestone was absent. 

 Recently, however, it has been shown that they can be dis- 

 tinguished by their graptolites, 2 and, further, that the Aymestry 

 limestone comes within the highest graptolite zone. 



Since, however, these beds are linked to the Wenlock Group 

 by the presence of graptolites and by a common assemblage of 

 trilobites and brachiopoda, Professor Lapworth has proposed to 

 unite them with the Wenlock Beds and to regard them as a 

 primary division under the name of Salopian. 



The highest beds referable to the Silurian System are a set of 

 shales, flagstones, and sandstones in which no graptolites are 

 found, but which contain remains of a remarkable group of 

 Arachnida the Eurypterida and also the bones of certain fish 

 which are the earliest vertebrates found in the British Isles. For 

 these beds the name Downtonian has been proposed, but Clunian 

 from Clun Forest, proposed by me in 1885 (Geol. Mag.), would in 

 some respects be a better name. 



