
ey Tl, Sucr. 92] ~ SILURIAN. | 683 
being specially abundant. In all 10 genera and 14 species of fishes 
have been recovered from the Ludlow rocks. The fish remains consist 
of bones, teeth, shagreen-like scales, plates, and fin-spines. They in- 
clude some plagiostomous (placoid) forms (Thelodus), shagreen-scales 
(Sphagodus), skin (the spines described under the name of Onchus being 
probably crustacean), and some ostracosteans (Cephalaspis, ’ Auchenaspis, 
and Péeraspis). 
In the typical Silurian region of Shropshire and the adjacent | 
counties, nothing can be more decided than the lithological evidence for 
the gradual disappearance of the Silurian sea, with its crowds of 
graptolites, trilobites, and brachiopods, and for the gradual introduction 
of those geographical conditions which brought about the deposit of the 
‘Old Red Sandstone. The fine grey and olive-coloured muds, with their 
occasional zones of limestone, are succeeded by bright red clays, sand- 
stones, cornstones, and conglomerates. The evidence from fossils is 
equally explicit. Up to the top of the Ludlow rocks the abundant 
Silurian fauna continues in hardly diminished numbers. But as soon 
as the red strata begin the organic remains rapidly die out, until 
at last only the fish and the large eurypterid crustaceans continue 
to occur. 
Turning now from the interesting and extremely important though 
limited area in which the original type of the Upper Silurian rocks is _ 
developed, we observe that whether traced northwards or south-west- 
wards the soft mudstones and thick limestones give way to hard slates, 
grits, and flagstones, among which it is scarcely possible sometimes even 
to discriminate what represents the Wenlock from what may be the 
equivalent of the Ludlow group. It is in Denbighshire and the adjacent 
counties that this change becomes most marked. ‘The Tarannon shale 
above described passes into that region of North Wales, where it forms 
the base of the Upper Silurian formations. It is covered by a series of 
. grits or sandstones which in some places are at least 3000 feet thick. 
These are covered by and pass laterally into hard shales, which are 
believed to represent parts of the true Wenlock group, perhaps even 
some portion of the Ludlow rocks. It is evident, however, that in spite 
of the wide extent over which these Silurian rocks of North Wales are 
spread, and the great thickness which they attain, they do not present 
an adequate stratigraphical equivalent for the complete succession in 
the original Silurian district. Instead of passing up conformably into 
the base of the Old Red Sandstone, as at Ludlow, they are covered by 
that formation unconformably. In fact they have been upturned, 
crumpled, faulted, and cleaved before the deposition of those portions of 
the Old Red Sandstone which he upon them. These great physical 
changes took place in Denbighshire when, so far as the evidence goes, 
there was entire quiescence in the Shropshire district; yet the distance 
between the two areas was not more than about 60 miles. These 
subterranean movements were doubtless connected with those more 
widely extended upheavals which converted the floor of the Silurian sea 
into a series of isolated basins, in which the Old Red Sandstone was 
laid down. 
In Westmoreland and Cumberland a vast mass of hard slates, grits, 
and flags, was identified by Sedgwick as of Upper Silurian age. These 
form the varied ranges of hills in the southern part of the Lake district 
