300 Dr. C. Ricketts—Oscillation of the Earth’s Crust. 
iferous Limestone has been ascertained to the amount of 1580 feet ;! 
but it may be considerably more, for the base can nowhere be deter- 
mined. Its thickness diminishes towards the north. From nine to 
twelve miles north-east of Settle, “in the vicinity of Arncliffe and 
Kettlewell, it is probably not less than 1000 feet thick.” To the 
west of Arncliffe six and twelve miles respectively, ‘under Penigent 
and Ingleborough it is from 400 to 600 feet thick, and along the 
Pennine chain from Kirkby Stephen northward a less thickness may — 
be assigned to it.” * 
Judging from such facts as these, there can be no question that the 
Carboniferous Limestone has been deposited on surfaces of unequal 
elevation, in valleys and gorges excavated in the older rocks, and 
that it has been almost throughout its whole extent precipitated 
slowly and equally, or been otherwise derived from the waters of the 
sea. The variation in thickness in different localities is very greatly 
owing to its having been deposited in the vacant spaces which 
formed these pre-Carboniferous valleys; at all events the evidence 
by itself is not sufficiently conclusive to decide that, though the 
accumulation has been greater in one place than in another, there- 
fore the subsidence has been caused by this increase of pressure. 
Though the subsidence has been almost entirely continuous during 
the whole period of the deposition of the limestone, the sea-bed 
formed was at no time situated at any great depth, for the valves of 
Mollusks are frequently separated, and the stems and joints of 
Encrinites have been washed from each other, as by the action of 
waves or currents. The water therefore having been shallow, a 
sufficiently correct base-line is afforded to determine the effects 
produced by the inundation of this submarine plain by large accumu- 
lations (derived from distant sources) which constitute the later 
Carboniferous rocks, whether a greater or less amount of subsidence 
occurred locally according to the amount deposited. 
In the Coalbrookdale district the thickness of the Millstone Grit, 
where it lies upon the Limestone at Steeraway, is 80 feet ;* a little 
beyond it rests upon Upper Silurian strata; whilst six miles further 
towards the south none has ever existed, the Coal-measures lying 
immediately upon Silurian rocks. At Llangollen according to Mr. 
Morton it increases in thickness to 723 feet. Including with it the 
Yoredales, it extends in North Derbyshire to more than 3000 feet, 
with a greatly varying thickness, being of that amount near Shef- 
field, but it is only 1500 feet near Belper, a distance of less than 30 
miles.? Beds of Coal occasionally recurring in these strata indicate 
low-lying land surfaces, posterior to the formation of which there 
has occurred subsidence with the accumulation many times greater 
in extent than its whole thickness near the southern margin. 
1 Mem. Geol. Survey, North Derbyshire, p. 18. 
2 Geology of Yorkshire, by John Phillips, F.R.S., part ii. p. 176. 
Geological Survey, Vertical Sections, No. 28. 
Under the local definition of Cefn-y-Fedw Sandstone, op. cit. p. 51. 
5 Fig. 37, Comparative Sections of the Millstone Grit and Yoredale Rocks, Mem. 
Geol. Survey, North Derbyshire, p, 139, ; , 
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