Chap. VI.] WENLOCK AND LUDLOW FOEMATIONS. 105 
CHAPTER VI. 
UPPER SILURIAN ROCKS. 
GENERAL CHARACTER OP 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 still 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 wilds 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 upon 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 Shropshire and Herefordshire ; but, as the 
same fossils prevail in all such cases, geological classification is unaffected 
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, all more or less affected 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.N".E. to S.S.W; in the rocks in various parts of Wales are, indeed, re- 
Wenlock and Ludlow district it is nearly If.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 confor- Government Survey, 
mity with the major axis of the great South-Welsh 
