THE UPPER SILURIAN PERIOD. 117 



distinguish the Lower Silurian. As compared, also, as regards 

 the total bulk of strata concerned, the thickness of the Upper 

 Silurian is generally very much below that of the Lower Silurian, 

 indicating that they represent a proportionately shorter period 

 of time. In considering the general succession of the Upper 

 Silurian beds, we shall, as before, select Wales and America as 

 being two regions where these deposits are typically developed. 

 In Wales and its borders the general succession of the 

 Upper Silurian rocks may be taken to be as follows, in ascend- 

 ing order (fig. 57) : 



(1) The base of the Upper Silurian series is constituted by 

 a series of arenaceous beds, to which the name of " May Hill 

 Sandstone " was applied by Sedgwick. These are succeeded 

 by a series of greenish-grey or pale-grey slates (" Tarannon 

 Shales"), sometimes of great thickness; and these two groups 

 of beds together form what may be termed the " May Hill 

 Group" (Upper Llandovery of Murchison). Though not very 

 extensively developed in Britain, this zone is one very well 

 marked by its fossils ; and it corresponds with the " Clinton 

 Group " of North America, in which similar fossils occur. In 

 South Wales this group is clearly unconf ormable to the highest 

 member of the subjacent Lower Silurian (the Llandovery 

 group) ; and there is a reason to believe that a similar, though 

 less conspicuous, physical break occurs very generally between 

 the base of the Upper and the summit of the Lower Silurian. 



(2) The Wenlock Group succeeds the May Hill group, and 

 constitutes the middle member of the Upper Silurian. At its 

 base it may have an irregular limestone ("Woolhope Lime- 

 stone"), and its summit may be formed by a similar but thicker 

 calcareous deposit (" Wenlock Limestone") ; but the bulk of the 

 group is made up of the argillaceous and shaly strata known as 

 the " Wenlock Shale. " In North Wales the Wenlock group is 

 represented by a great accumulation of flaggy and gritty strata 

 (the "Denbighshire Flags and Grits"), and similar beds (the 

 " Coniston Flags" and " Coniston Grits") take the same place 

 in the north of England. 



(3) The Ludlow Group is the highest member of the Upper 

 Silurian, and consists typically of a lower "arenaceous and shaly 

 series (the "Lower Ludlow Rock"), a middle calcareous 

 member (the " Aymestry Limestone"), and an upper shaly and 

 sandy series (the "Upper Ludlow Rock"), and " Downton Sand- 

 stone"). At the summit, or close to the summit, of the Upper 



