166 Dr. C. Ricketts—Changes in the Earth's Crust. 
depth greater than that of the present valley; and in width extend- 
ing from Llansantfraid Glyn Ceiriog, to the Cyrn-y-brain mountain, 
a distance from each other of seven miles in a direct line. Between 
these two points, though the strata have been deeply cut into, by the 
formation of the present valley of the Dee, no traces of the older 
rocks are discovered. Upon each side of this assumed valley, 
especially where the Upper Silurian strata abut against the older or 
Bala rocks, bendings and foldings have taken place, and slaty- 
cleavage has been developed, whilst at Llangollen, half-way between 
the two localities, the beds are seen, in the course of the river Dee, 
lying in a horizontal position. It has been already shown that such 
foldings could not have resulted from compression produced solely 
by subsidence to the extent indicated by the thickness of the deposit. 
This may be considered to be further confirmed in the same locality, 
where a repetition of the circumstances has so far taken place, that a 
deep valley, formed by the denudation of this Wenlock rock, had its 
bottom covered with the Red Basement beds of the Carboniferous 
system, and was subsequently buried underneath a deposit of 1025 
feet’ of limestone, which, instead of folding, has a dip of 15° only 
to the N.E., passing beneath 3000 to 4000 feet? of later Carbon- 
iferous strata. There are essential] differences in the character of the 
deposits in the two periods; the Silurian, consisting of fine mud, 
which, long after its deposition, would remain in a plastic state and 
readily yield when subjected to irregular pressure, whilst the lime- 
stone would soon consolidate and become rigid.’ 
In the Ingleborough district the Lower and Upper Silurians are 
greatly bent and contorted, and the muddy deposits are also 
cleaved ;* they were greatly denuded, and formed hills and valleys, 
previous to the deposition upon them of the Carboniferous lime- 
stone, ‘the lowest beds of which,” as Playfair (§ 197) remarked, 
“contain in them many fragments of stone which, on comparison, 
resemble exactly the schistus underneath.” A thickness of lime- 
stone, amounting to 600 feet, rests on the basset edges of the 
Silurians, and this is surmounted, as seen in the mountain peaks of 
1 The Carboniferous Limestone and Cefn-y-fedw Sandstone of North Wales by 
G. H. Morton, F.G.S., p. 21. 
2 The Coal-fields of Great Britain, by Professor E. Hull, 2nd ed. p. 99. 
3 | have seen at least ten instances where the Carboniferous Limestone during the 
period of its formation had been locally raised above the sea-level, and had thus been 
exposed for a time to atmospheric action and erosion. In eight of the examples thin 
beds of coal were formed, either resting immediately on the sculptured surtace, or on 
a bed of clay which covered it; the thickest amounts to about a toot, at Ingleton; 
others varied down to a mere film of carbonaceous matter in the clayey deposit. 
Subsidence taking place, the deposition of the limestone again progressed to a great 
extent. The methods of weathering go to prove that the limestone had become 
consolidated previously to its elevation and erosion ; in some cases shrinkage joints 
are present, and are filled with calcite, which stands out in relief on the eroded sur- 
face; the condition of these small joints indicates that the limestone must have 
become solidified before being raised above the sea-level, prior to the formation of the 
coaly beds. In some instances angular or weathered fragments of limestone are 
embedded in the clayey deposit which fills hollows in the surface of the limestone. 
4 They have been described in an Essay by Professor T. McK. Hughes, *‘ On the 
Break between the Upper and Lower Silurians of the Lake District,’ Gro, Mac. 
Vol. IV. p. 346. 
