Dr. C. Ricketts—Oscillation of the Earth's Crust. ool 
Similar variation in the thickness of deposits occurs also in the 
Coal formation. Coalbrookdale again affords an example where the 
accumulations and the changes of level are small compared with 
other districts; probably because it was situated at a distance from ~ 
the sources from which the detritus was conveyed, and therefore 
could not receive the same amount of deposit. The strata in which 
the profitable beds of coal are situated range from “Little Flint 
Coal,” situated immediately above the “ Farewell Rock ” (Millstone 
Grit) to the “Top Coal,” and amount to only 180 feet; the gross 
average thickness of the beds of coal being about 40 feet,’ that of 
the beds of workable coal according to Professor Hull 27 feet; the 
entire thickness of the Coal-measures amounts to 1200 feet.2 In 
Denbighshire the Middle Coal-measures, the only ones worked, have 
a thickness of 800 feet, with from 27 to 30 feet of workable coal. 
The Lower Measures amount to 1000 feet, with several coal-seams 
varying from two to three feet. The aggregate thickness of these 
Coal-measures is about 38000 feet. Again a very great increase is 
observed in the Coal-measures of the Lancashire district, where, in- 
cluding the Gannister beds, they attain a maximum thickness of 
7300 feet.‘ | 
Taking Leicestershire as a starting-point, the different series of 
Upper Carboniferous strata attain a very moderate thickness; but a 
great and progressive increase occurs when passing thence in a 
north-westerly direction to North Lancashire. Professor Hull has 
demonstrated this in the following Table of 
‘¢CoOMPARATIVE VERTICAL SECTIONS OF THE CARBONIFEROUS STRATA FROM 
Nortu LANCASHIRE TO LEICESTERSHIHE. 
N.N.W. Gok 
Burnley dis- Mottram dis- | North Stafford- | : sagen 
Hae i: i fee! 2 “s ses oe Leicestershire. 
Gonlemeasures © ..62...0< 6.0. 8460 7635 6000 3000 
NVinnShomesGaribeaeesseeece ce: 5500 2500 500 50 
BY(OneU MeSmyasesc nce tse acess 4675 2000 2300 50 
18,635 12,135 8800 3100.’ 
Commenting on the above sections, Professor Hull remarks, that 
“it will be observed the beds which attained so prodigious a 
development in North Lancashire dwindled down to one-sixth of 
their volume in Liecestershire.”® The Coal series varies consider- 
ably in limited areas in different localities, and there is in Lancashire 
a general increase of thickness of the sedimentary materials, such 
as .sandstones and shales, towards the N.N.W. Thus the same 
coal-seams are farther apart at St. Helens than at Prescot, and at 
Wigan than at St. Helens.* This difference in the thickness of 
the Coal-measures occurring within restricted areas has been con- 
firmed by Messrs. De Rance and Strahan, who prove the amount 
1 Geological Survey, Vertical Sections, No. 23, 
2 The Coalfields of Great Britain, by Edward Hull, F.G.S., second edition, p. 96. 
3 Hull, op. cit. p. 99. 4 Hull, op. cit. p. 123. 
5 Quart. Journ. Geol. Soc. vol. xxiv. p. 322. 
6 Hull, Coal-fields of Great Britain, p. 123. 
