Chap. VI.]            WENLOCK AND LUDLOW FORMATIONS.                      105
 

CHAPTER VI.
UPPER SILURIAN ROCKS.

general character of the upper SILURIAN   ROCKS, AS   DIVIDED   INTO   THE  WENLOCK AND

LUDLOW   formations.----THE WENLOCK   FORMATION   OF   SHALE  AND   LIMESTONE, WITH   ITS

CHIEF FOSSILS, DESCRIBED IN ASCENDING ORDER, FROM THE SHALE WITH WOOLHOPE LIME¬
STONE TO THE WENLOCK OR DUDLEY LIMESTONE INCLUSIVE.

Examples have already been adduced to indicate the order in which the
Lower Silurian rocks are overlain by the higher division of the system;
and we have now, therefore, to consider the mineral characters and fossil
contents of the latter as composed, in ascending order, of the Wenlock and
Ludlow formations.

As the older schists and slates were assuredly at one period nothing
more than marine mud, finely laminated, so is it stUl more apparent that
such was the former state of the greater portion of the Upper Silurian;
for even at the present day the latter is composed of materials for the most
part similar to those of the older slates, though in a softer and less cohe-
herent state.

Whether these argillaceous masses be examined in the wUds of Radnor
Forest and the eastern parts of Montgomery, in the western parts of Shrop¬
shire (Long Mountain), or in many tracts of South Wales (see Map), they
present the uniform * facies ' of a thick, yet finely laminated, dark, dull-
grey shale, in which hard stone of any strength or persistence is the rare
exception.    Their dominant character, in short, is that of ' mudstone.'

Ranging chiefly from S.W. to N.E., they rest conformably upoD lower
rocks, in numerous undulations *. Looking to the whole region in which
these rocks are laid down upon the annexed Map (see colour No. 5), there
are considerable tracts of North Wales where the lowest members of the
Wenlock formation possess mineral characters which distinguish them from
the types originally described in Shro|)shire and Herefordshire; but, as the
same fossils prevaU in aU such cases, geological classiflcation is unaff'ected
by the lithological variations. The base of the deposit in Wales often
consists, as previously stated, of sandstones or grits, with shales and flag¬
stones, aU more or less afl'ected by a slaty cleavage. These have been
termed ^ Denbigh grits ;'   but as the sandy beds thicken and thin out

* The strike of the same deposits varies in dif-     coal-field (see Map).    The apparently conform-

ferent districts.     Thus in North Wales the pre-     able undulations of the Lower and Upper Silurian

valent strike is nearly N.IS'.E. to S.S.W; in  the     rocks in various parts of Wales are, indeed, re-

Wenlock and Ludlow district it is nearly N.E. to     presented in the diagrams already given, pp. 102,

S.W.; whilst in South Pembroke all the Silurian     103, as taken from the published Sections of the

rocks range from W. by N. to E. by S., in confer-     Grovernment Survey,
mity with the major axis of the great South-Welsh