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 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.
Pages:
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198