Chap, v.] INTERMEDIATE CHARACTER OF LLANDOVERY ROCKS.       85
 

CHAPTEK V.

LLANDOVERY ROCKS (TRANSITION FROM LOWER TO UPPER

SILURIAN).

THIS FORiMATION SHOWN TO BE OP INTERMEDIATE CHARACTER, CONTAINING BOTH LOWER
AND UPPER SILURIAN FOSSILS, WITH SOME PECULIAR TYPES.—ASCENDING ORDER OF THE
WHOLE GROUP NEAR LLANDOVERY IN SOUTH WALES, WHERE MOST DEVELOPED.—THE
UPPER PORTION ALONE EXHIBITED IN RADNORSHIRE, SHROPSHIRE, HEREFORDSHIRE, THE
MALVERN HILLS, MAY HILL, TORTWORTH, THE LICKEY HILLS, ETC.—TARANNON SHALES,
OR  BASE   OF  THE   UPPER   SILURIAN   IN   NORTH   WALES.

In my earher pubhcations, the whole group of strata to which attention
is now invited was placed at the summit of the Lower Silurian rocks. The
progressive researches and labours of my cotemporaries, however, have
for some time led me to admit that, whilst the lower and larger portion
of this formation is related, through numerous organic remains, to the in¬
ferior half of the system, the higher member, though also containing some
of the lower types, is, by many of its fossils, more naturaUy connected
with the Upper SUurian *. At the same time, the formation is eminently
characterized throughout by certain Pentameri peculiar to it, which are
not known either in the subjacent or superjacent deposits, and also by
some species of other shells which never rise into the Wenlock (or true
Upper SUurian) rocks. A short chapter is therefore devoted to the con¬
sideration of this group, showing its intermediate character and varied
development in different locahties.

Though not recognizable in the northern or central parts of the SUurian
region, the inferior member of the formation is fully exposed in extensive
tracts of South Wales to the north and west of LlandeUo, particularly near
Llandovery, where it was formerly described in detail (Sil. Syst. p. <351).
Let us first, then, consider the nature and contents of these rocks as seen in
ascending order in the last-mentioned tract, where both their members are
exposed, and afterwards treat of them in certain districts where the lower
member of the group is wanting and the uppermost subdivision alone is
visible.

In the first edition of this work, all these rocks were termed ' Upper
Caradoc ;' but that term was set aside in the last edition. The local name
of ' Llandovery Eocks ' was then taken, from the district where clear phy¬
sical relations are seen of the transition from the Caradoc formation beneath,

* This last feature was first pointed out by memoir on the May Hill Sandstone (Phil. Mag.
Professor Sedgwick and Professor M'Coy in their    ser. 4. vol. ^iii. 1854, p. 301).