Dr. C. Ricketts—Osciliation of the Earth’s Crust. 3849 
great change occurred, whereby waters, bringing down soluble 
silicate, mud, sand, and pebbles, were diverted into this area; the 
distribution of which produced, according to the conditions prevail- 
ing in different periods and localities, in the first place siliceous and 
carbonaceous materials characterizing the Upper Limestone, after- 
wards the varieties of strata characterizing the Yoredales and Mill- 
stone Grit, and at a later period the Coal-measures. 
A modern event which must produce effects comparable with,—at 
least causing as great a contrast as the changes which occurred in the 
Carboniferous strata above the Limestone,—took place about 1552, 
when the muddy waters of the Whang-Ho were diverted from the 
Yellow Sea into the land-locked Gulf of Pe-Che-Lee, the present 
outlet being removed a distance of not less than 640 miles along 
the coast-line, around the rocky promontory of Shan-tung (Hast 
Mountain).' 
A very great variation in the thickness of the Carboniferous Lime- 
stone occurs in different places, and often within very short distances. 
In the Coalbrookdale district the southern boundary of this forma- 
tion extends no farther than Little Wenlock, near which village, at 
Oldfield Quarry, it is 25 feet thick; and a little further northward, 
at Steeraway, it is 40 feet,? but does not exceed 100 feet. It is 
evident the Carboniferous Sea did not extend over the Silurian 
rocks to the south, or over those of the Longmynd towards the 
west; for any strata resting upon them and belonging to that era 
are referred to the Coal-measure series. | 
At Llanymynech, 25 miles north-west of Coalbrookdale, the 
nearest locality where the Carboniferous Limestone now exists, the 
precipitous escarpment of Llanymynech Hill rises from a basement 
of Wenlock shale. Mr. G. H. Morton estimates the thickness of 
this mass of limestone at 450 feet.2 This gets thinner towards the 
north, and at Grug-fryn, nine miles distant, it exists along such a 
narrow space, resting on Wenlock shale and having sandstone above, 
that the limestone seems to be not more than 100 feet, or perhaps 
only 50 feet in thickness.‘ 
Near Llangollen, where it overlies beds of sandstone and con- 
glomerate, the Carboniferous basement beds previously alluded to, 
the limestone forms the magnificent escarpment of the Hglwyseg 
ridge, and is exposed throughout its whole extent. Its entire thick- 
ness according to Mr. Morton, is 1200 feet. This forms a remark- 
able contrast to the exposure on the opposite bank of the River Dee. 
Within a distance of four miles, at Fron-y-Cysyllte, where the lime- 
stone rests on Wenlock shale, there are wanting no less than 873 
feet of the lower strata as developed in the Hglwyseg ridge.°* 
In North Derbyshire near Buxton the thickness of the Carbon- 
1 The Double Delta of the Whang-Ho, by Samuel Mossman, Geographical 
Magazine, vol. v. p. 92. 
2 Geological Survey, Vertical Sections, No. 23. 
3 The Carboniferous Limestone and Cefn-y-fedw Sandstone between Llanymy- 
nech and Minera, by G. H. Morton, F.G.S., p. 122. 
+ Morton op. cit., p. 81. ® Morton, op. cit. pp. 39 and 75. 
