ON THE PROBABLE ORIGIN AND AGE OF THE SUN. 



353 



10,000 feet. 1 Prof. H. D. Rogers found in the 

 Appalachian coal-fields faults ranging from 5,000 

 feet to more than 10,000 feet of displacement. 



There are other modes than the foregoing, by 

 means of which geologists are enabled to measure 

 the thickness of strata which may have been re- 

 moved in places off the present surface of the 

 country, into the details of which I need not here 

 enter. But I may give a few examples of the 

 enormous extent to which the country, in some 

 places, has been found to have been lowered by 

 denudation. 



Prof. Geikie has shown 2 that the Pentlands 

 must at one time have been covered with upward 

 of a mile in thickness of Carboniferous rocks 

 which have all been removed by denudation. 



In the Bristbl coal-fields, between the river 

 Avon and the Mendips, Prof. Ramsay has shown 3 

 that about 9,000 feet of Carboniferous strata have 

 been removed by denudation from the present 

 surface. 



Between Bendrick Rock and Garth II ill, South 

 Glamorganshire, a mass of Carboniferous and Old 

 Red Sandstone, of upward of 9,000 feet, has been 

 removed. At the vale of Towy, Caermarthen- 

 shire, about (5,000 feet of Silurian and 5,000 feet 

 of Old Red Sanstone — in all about 11,000 vertical 

 feet — have been swept away. Between Llan- 

 dovery and Aberaeron a mass of about 12,000 

 vertical feet of the Silurian series has been re- 

 moved by denudation. Between Ebwy and the 

 forest of Dean, a distance of upward of twenty 

 miles, a thickness of rock varying from 5,000 to 

 10,000 feet has been abstracted. 



Prof. Hull found 4 on the northern flanks of 

 the Pendle Range, Lancashire, the Permian beds 

 resting on the denuded edges of the Millstone 

 Grit, and these were again observed resting on 

 the Upper Coal-measures south of the Wigan 

 coal-field. Now, from the known thickness of 

 the Carboniferous series in this part of Lanca- 

 shire, he was enabled to calculate approximately 

 the quantity of Carboniferous strata which must 

 have been carried away between the period of the 

 Millstone Grit and the deposition of the Permian 

 beds, and found that it actually amounted to no 

 less than 9,900 feet. He also found in the vale 

 of Clitheroe, and at the base of the Pendle Range, 

 that the Coal-measures, the whole of the Millstone 



1 Safford's " Geology of Tennessee," p. 309. 



2 Memoir to Sheet 32, " Geological Survey of Scot- 

 land." 



3 "Denudation of South Wales;'" "Memoirs of Geo- 

 logical Survey," vo\ i. 



* Quarterly Journal of the Geological Society, vol. 

 sxiv., p. 323. 



23 



Grit, the Yoredale series, and part of the Car- 

 boniferous Limestone, amounting in all to nearly 

 20,000 feet, had been swept away — an amount of 

 denudation which, as Prof. Hull remarks, cannot 

 fail to impress us with some idea of the prodi- 

 gious lapse of time necessary for its accomplish- 

 ment. 



In the Nova Scotia coal-fields one or two miles 

 in thickness of strata have been removed in some 

 places. 1 



It may be observed that, enormous as is the 

 amount of denudation indicated by the foregoing 

 figures, these figures do not represent in most 

 cases the actual thickness of rock removed from 

 the surface. We are necessitated to conclude 

 that a mass of rock equal to the thickness stated 

 must have been removed, but we are in most 

 cases left in uncertainty as to the total thickness 

 which has actually been carried away. In the 

 case of a fault, for example, with a displacement 

 of (say) one mile, where no indication of it is 

 seen at the surface of the ground, we know that 

 on one side of the fault a thickness of rock equal 

 to one mile must have been denuded, but we do 

 not know how much more than that may have 

 been removed. For anything which we know to 

 the contrary, hundreds of feet of rock may have 

 been removed before the dislocation took place, 

 and as many more hundreds after all indications 

 of dislocation had been effaced at the surface. 



But it must be observed that the total quan- 

 tity of rock which has been removed from the 

 present surface of the land is evidently small in 

 proportion to the total quantity removed during 

 the past history of our globe. For those thou- 

 sands and thousands of feet of rock which have 

 been denuded were formed out of the waste of 

 previously-existing rocks, just as these had been 

 formed out of the waste of yet older rock-mass- 

 es. In short, as a general rule, the rocks of one 

 epoch have been formed out of those of preced- 

 ing periods, and go themselves to form those of 

 subsequent epochs. 



In many of the cases of enormous denudation 

 to which we have referred the erosion has been 

 effected during a limited geological epoch. We 

 have, for example, seen that upward of a mile in 

 thickness of Carboniferous rock has been de- 

 nuded in the area of the Pentlands. But the 

 Pentlands themselves, it can be proved, existed 

 as hills, in much their present form, before the 

 Carboniferous rocks were laid down over them ; 

 and as they are of Lower Old Red Sandstone 

 age, and have been formed by denudation, they 

 1 Lyell's " Student's Manual," chap, xxiii. 



