Correspondence. 327 
with 500 or more feet of Carboniferous rocks, will give a downthrow 
to the NW. of 3,000 or 4,000 feet to the fault there. Now, suppose 
we imagine this dislocation to be reduced (to borrow a term from the 
surgery) by the simple process of lifting up the Tarannon Shale on 
the downthrow side of the fault, and putting it on the same level 
as the corresponding beds on the Corwea side of the fault, as sug- 
gested in fig. 2. 
Fig. 2. 
Caeran Limestone of 
Crwyni. Wenlock Shales. Hafod y Calch. Corwen. 
Dislocation of 
3,500 feet (reduced). 
Tarannon Shale. 
We should then have a hill 3,000 or 4,000 feet in height capped by 
the Carboniferous Limestone; and anyone standing an that hill and 
looking to the east would see the limestone cliffs of the Eglwysegle 
rocks near Llangollen, with the ends of the beds facing right at 
him. Produce the beds about Llangollen with their mean rise of 
about 5° towards Corwen, and they would just be about 4,000 feet 
above the level of the country there. 
It is therefore a by no means improbable supposition that the 
preservation of that little patch near Corwen is due to the joint 
action of the downthrow of the fault, and a local busin-shaped de- 
pression of the beds there. An undulation.in the beds at that par- 
ticular spot perhaps enabled the fault to bring down higher beds 
than it did just north of it. 
The effect was, that the patch of limestone was there let down to 
such a low level into the earth, that the denuding forces which have 
ever since been at work have not yet destroyed it. It has been 
spared as if to prove to us that at the time when the great fault was 
produced, the limestone spread over the whole area. > 
Now when was that great fault produced? and when did the chief 
part of the destruction and removal of rock take place ? The answer 
to these questions will entirely absolve us from the necessity of look- 
ing for the materials, and especially from searching for them in the 
Drift of the Glacial period. 
Whenever the fault commenced, its production was finished before 
the time of the deposition of the New Red Sandstone. In like man- 
ner, although denudation has been taking place ever since the first 
elevation of the rocks above the sea, yet by far its greatest amount 
occurred before the deposition of the New Red Sandstone. 
Let anyone walk from Corwen up the Nant Morwymon, through 
