674 STRATIGRAPHICAL GEOLOGY. [Book VI. | 
comparatively flat, soft, and unaltered, consisting mainly of somewhat — 
incoherent sandy mudstone and shale, with occasional bands of limestone. ~ 
But as these rocks are followed into North Wales, they are found to swell — 
out into a vast series of grits and shales, so like portions of the hard altered — 
Lower Silurian rocks that, save for the evidence of fossils, they would — 
naturally be grouped as part of that more ancient series. In Westmore- 
land and Cumberland, and still further north in the border counties of 
Scotland, also in the south-west of Ireland, it is the North Welsh type 
which prevails, so that in Britain the general lithological characters 
and minute paleontological subdivisions ascertained in the original 
Silurian district are almost confined to that limited region, while over the 
rest of the British area for thousands of square miles the hard sandy and 
shaly type of North Wales is prevalent. . 
Taking first the Silurian tract of the west of England, and 
the east and south of Wales, we find a decided unconformability 
separating the Lower from the Upper Silurian deposits. In some 
places the latter steal across the edges of the former, group after group, 
till they lie directly upon the Cambrian rocks. Indeed, in one district 
between the Longmynd and Wenlock Edge, the base of the Upper 
Silurian rocks is found within a few miles to pass from the Caradoc 
group across to the Lower Cambrian rocks. It is evident, therefore, 
that in the Welsh region very great disturbance and extensive denuda- 
tion preceded the commencement of the deposition of the Upper Silurian 
rocks. As Sir Andrew C. Ramsay has pointed out, the area of Wales, 
previously covered by a wide though shallow sea, was ridged up into a 
series of islands, round the margin of which the conglomerates at the 
base of the Upper Silurian series began to be laid down. This took 
place during a time of submergence, for these conglomeratic and sandy 
strata are found creeping up the slopes and even capping some of the 
hills, as at Bogmine, where they reach a height of 1150 feet above the 
sea.’ The subsidence probably continued during the whole of the 
interval occupied by the deposition of the Upper Silurian strata, which 
were thus piled to a depth of from 3000 to 5000 feet over the disturbed 
and denuded platform of Lower Silurian rocks. 
Arranged in tabular form, the subdivisions of the Upper Silurian 
rocks of Wales and the adjoining counties of England are in descending 
order as follows : 
Base of Old Red Sandstone. 
Tilestones. 
Upper Ludlow Rock. 
Aymestry Limestone. 
Lower Ludlow Rock, 
Wenlock or Dudley Limestone 
3. Ludlow group. 
i é Denbighshire 
ayer Wenlock Shale. ; ; . ; & 
2, Wenlock group Woolhope or Barr Limestone and Shale . {~ N et 2 
Tarannon Shale. , . ‘ . orth Wales. 
1. Upper Llan- 
dovery group fay Hill Sandstones. 
Lower Llandovery Rocks. 
1. Upper Llandovery group.—May Mill Sandstones.—The position of 
these rocks as the true base of the Upper Silurian groups was first 
shown in 1853 by Sedgwick, who named them the May Hill Sandstones 
from the locality in Gloucestershire where they are so well displayed. 
* Ramsay, Physical Geology and Geography of Britain, p. 91. 

