THE UPPER SILURIAN PERIOD. PxS 
CHAPTERS es 
PHEPUPPER: SILURIAN BERIOD: 
Having now treated of the Lower Silurian period at consider- 
able length, it will not be necessary to discuss the succeeding 
group of the Upper Silurian in the same detail—the more so, 
as with a general change of sfecies the Upper Silurian animals 
belong for the most part to the same great ¢ypes as those which 
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 All 
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 unconformable to the highest 
member of the subjacent Lower Silurian (the Llandovery group); 
and there is reason to believe that a similar, though less con- 
spicuous, 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 bya 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 
