CLASSIFICATION OF PALAEOZOIC ROCKS OF THE BRITISH ISLES. 305 
3. Silurian ( Upper Silurian , Murchison). 
Though this name was intended by Murchison to include, 
perhaps, all the fossiliferous rocks below the Devonian, it has 
been quite evident for a long time that the restricted meaning 
in which it is here used is the only way to get rid of much 
confusion in geological nomenclature. The groups here in- 
cluded in it are, in ascending order, Llandovery, Wenlock, and 
Ludlow. 
Llandovery group . — The name was given to this group by Sir 
It. Murchison in consequence of the great development of rocks 
of this age near the town of Llandovery, S. Wales. It is sup- 
posed to attain a thickness, there and in Pembrokeshire, of 
over 2000 feet ; whilst it is well known that in some other 
areas in Wales it thins out to about as many hundred feet. 
The physical break between this group and the underlying Bala 
has been frequently referred to by Sedgwick, Bamsay, and others, 
and there can be no doubt that it is the most important one in 
the Lower Palaeozoic rocks. To a great extent, however, it must 
be considered to have been produced by local influences. For 
whilst in the Longmynd and other areas there is ample evi- 
dence now to show that there was dry land at the time, there is 
equally convincing proof that marine conditions continued dur- 
ing the sanrn period in Cardiganshire and Pembrokeshire. In 
Sect. 3, p. 30$, the Lower Llandovery rocks rest conformably upon 
the Bala beds. The rocks which compose this group vary con- 
siderably in different areas, but may be said to consist chiefly 
of conglomerates, sandstones, shales, and slates. It is usually 
divided into lower and upper portions, — the latter being also 
called frequently the 4 May-Hill Sandstone series.’ The 
so-called ‘Tarannon shales’ and ‘ Denbighshire grit and flag 
series/ are probably on the horizon of the upper members of 
this group. In the last-mentioned series I have recently dis- 
covered the earliest indications of land-plant remains hitherto 
found in Britain. They are all in a fragmentary condition 
and must have been carried into their present positions from 
some neighbouring lands by floods. These beds have yielded 
to Prof. Lapworth, in Scotland, a rich fauna of Graptolites, 
the prevailing family in the lower beds being the Diplograptidae, 
whilst in the upper beds the Monograptidae are dominant. 
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Upper. 
Lower. 
Conglomerates, Sandstones, Shales, and Slates. 
Conglomerates, Sandstones, and Shales. 
Wenlock Group . — This group has a considerable thickness, 
NEW SERIES, VOL. V. NO. XX. X 
