106 
SILUEIA. 
[Chap. VI. 
rapidly, they are simply to be viewed as local features among the great 
masses of shale (4 a of the Map)*. In fact, they are simply sediments 
which were sandy in the portion of a former sea that covered the tracts 
now constituting Denbighshire, and were muddy or argillaceous in more 
eastern areas. Passing, however, from such varied mineral conditions (for 
at Marloes Bay in Pembrokeshire the strata of this age are again sandy), 
it may be affirmed that usually the Upper Silurian rocks of Wales have 
been only so far affected by slaty cleavage as to leave them in that dis- 
jointed, incoherent state of mudstone (provincially ( rotch '), so useless to 
the mason and miner, and so cold and profitless to the agriculturist. In 
all such tracts, where the subdividing limestones are absent, or feebly in- 
dicated, it is only by close observation of the imbedded fossils that the 
formations, so clear and typical in other parts, can be recognized. 
When, on the contrary, we follow the same deposits from Wales to the 
exemplar tracts of Shropshire and Herefordshire, where the Upper Silurian 
rocks were classified, and their order, character, and fossils first described, 
Ave find them diversified by interpolated courses of limestone — much cal- 
careous matter being disseminated, both in nodules and in flagstones. 
With such additions to the richness of the subsoil, so welcome to the 
proprietor, the geologist usually discovers a greater abundance of fossil 
animal remains than in the sterile siliceous or argillaceous strata, cotem- 
poraneously formed, in the western tracts ; whilst by observing the order 
of superposition, and by tracing the divisionary limestones, he reads off 
the order of the beds, and chronicles with precision the succession of their 
respective fossils. 
In this way the Upper Silurian rocks are seen to consist, as a whole, of 
the two formatioDS to which I assigned the names of 'Wenlock' and 
' Ludlow,' each of these being subdivided in the manner expressed in 
this woodcut. 
General Order of the Upper Silurian Rocks included between the Upper 
Llandovery (May Hill) Sandstone and the Old Red Sandstone. 
c. Upper Llandovery rock, occasionally a limestone, but often a pebbly sandstone. 
d 1 . Shale, with Lower Wenlock or Woolhope limestone. d 2 . Wenlock shale. 
d 3 . Wenlock limestone. e l . Lower Ludlow, e 2 . Middle Ludlow or Aymestry lime- 
stone. e 3 . Upper Ludlow and Tilestone. /. Bottom of Old Eed Sandstone. 
The inferior member of the Wenlock formation, which rests on the Upper 
Llandovery rocks, c, as seen in Shropshire and parts of Wales, is chiefly a 
mass of dull, argillaceous, dark-grey shale, rarely if ever micaceous, and 
* Although the Denbigh grits are marked in the by implied,— the strata defined by the letters a, b, 
Map as 4a, to distinguish them from 4b, the Wool- c being simply lithological varieties of the lower 
hope limestone, and 4c, Wenlock shale, the reader portion of the Wenlock formation usually deve- 
is not to infer that any difference of age is there- loped as shale. 
