s 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 unconformable to the
highest member of the subjacent Lower Silurian (the Llandovery
group); and there is 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 Limestone"), 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 Sandstone"). At the summit, or close
to the summit, of the Upper Ludlow, is a singular stratum only a
few inches thick (varying from an inch to a foot), which contains
numerous remains of crustaceans and fishes, and is well known
under the name of the "bone-bed." Finally, the Upper Ludlow rock
graduates invariably into a series of red sandy deposits, which,
when of a flaggy character, are known locally as the "Tile-stones."
These beds are probably to be regarded as the highest member
of the Upper Silurian; but they are sometimes looked upon as
passage-beds into the Old Red Sandstone, or as the base of this
formation. It is, in fact, apparently impossible to draw any
actual line of demarcation between the Upper Silurian and the
overlying deposits of the Devonian or Old Red San