40 THE DENUDATIONS OF NORTH WALES. 
were that were first deposited and subsequently denuded away, 
if we went to the right place to look for them. Accordingly on 
travelling southwards to Ludlow and Herefordshire and parts of 
South Wales, we find an apparent sequence of rocks of enormous 
thickness intervening between the Carboniferous system and 
the equivalents of our Upper Silurian rocks, nearly, if not 
quite, filling up the gap we observe in our own district. The 
strata consist of the newest Silurian beds, including three sub- 
divisions known as the Ludlow Beds, which graduate upwards 
into the ‘‘ Passage Beds” or Tilestones. These again are said to 
pass up into the Old Red Sandstone of Herefordshire, a portion 
of which may probably belong to the Carboniferous system, and 
represent our Basement Beds. The whole mass of strata here 
seen, which are unrepresented in North Wales, is said to 
attain a thickness of about 10,000 feet. 
In accounting for the absence of all these beds in North 
Wales, we might suppose, in the first place, that they had never 
been deposited there at all, that part having become land, while 
the Nilurian sea still overspread the region to the south; or, in 
the second place, that they were deposited over both areas alike, 
but by unequal elevation had been subjected to greater waste in 
the one area than in the other. On the first of these suppositions 
we are met with the following difficulties :—There is no evidence 
in the Ludlow Beds of their having been deposited against a 
shore-line such as must have bounded the North Wales area on 
this hypothesis; and, further, how came the Silurian rocks of 
North Wales to be cleaved and metamorphosed if they were 
never covered with the later deposits of the period? On the 
other hand, it is in favour of the second hypothesis that near the 
Lake District, where the Upper Silurian re-emerge from beneath 
the Carboniferous rocks, they present a considerable similarity, 
both in sequence and fossils of the beds, to those of Central 
Wales, as though they had formed part of one continuous 
deposit. The difficulty in accepting this theory lies in the vast 
amount of denudation which it implies at a very early period, 
namely, before the commencement of the Carboniferous Period. 
And though we might be prepared, in observing such an 
unconformity as that shewn in the banks of the Clywedog, to 
graut that the Silurian strata must have been deeply denuded, 
yet it would have been a large assumption that so great a mass 
had been clean swept away, had no further evidence been 
forthcoming. 
Before detailing this evidence it is necessary to give a short 
sketch of the beds as they occur in Central Wales and in 
Westmorland. In the former the Wenlock Shale, the highest 
member of the Upper Silurian that is present in North Wales, 
passes up into grey micaceous sandstones and shales with a 
band of very fossiliferous limestone, which are collectively 
known as the Ludlow series. his series graduates upwards 
into a thick mass of red beds, which, by their fossils, are allied 
