226 Prof. Hughes, Oriticism of the Geological [Oct. 30, 



Survey Sections, we find that where the sediment is small we have 

 about 32,500 feet, but different members swell out in areas where 

 sediment was more abundant at the time they represent, or depres- 

 sion more rapid, and there we get 63,000 feet for the Cambrian 

 and Silurian. In these calculations we are less likely to obtain 

 wrong results if we take all our measurements from as nearly as 

 possible the same district, so as to avoid errors arising out of mis- 

 taken determination of boundaries or compensating accession of 

 thickness. 



Here then taking the lowest estimate we have evidence of the 

 descent of a land surface to depths greater than the deepest abyss 

 of ocean. The fact that these beds are now above sea-level and 

 exposed to observation is sufficient proof that a great elevation in 

 due course followed the subsidence we have just described. But 

 the upheaval did not stop there. The Cambrian and Silurian 

 Rocks were thrown into great folds, the tops of which were cut off in 

 time by denudation. That 30, 40, 50, or 60 thousand feet of sedi- 

 ment should be uplifted into arches above sea-level implies earth 

 movement on a scale amply sufficient to account for any climatal 

 changes possible within the periods recorded in the rocks. But 

 when denudation kept pace with upheaval, there would not be any 

 very high lands to produce intense cold, and when no traces of 

 glacial conditions are found it may be that the land was reduced 

 by denudation as fast as uplifted within its influence. The result 

 of all the combined operations is that when the area again subsides, 

 the new sediment is laid upon the planed-down surface of the older 

 rocks, and a measure of the time and extent of the movements may 

 be obtained by restoring the curves of the truncated strata. 



I have applied this method to the examination of the break 

 between the Silurian and the Carboniferous Periods in the north 

 of England, confining my observations within an area so small that 

 it is not necessary to make allowance for the thinning out of the 

 strata in any direction, and I found that strata the aggregate thick- 

 ness of which amounted to 27,000 feet were thrown into folds above 

 sea-level and remained there long enough to be all planed off at sea- 

 level. The height to which the crest of these folds must have risen 

 if they had not been destroyed pari passu with the upheaval could 

 not be less than 27,000 feet and may have been much more. 



The time for the reversal of the direction of movement came, 

 and throughout long ages shallow^ water conditions were maintained 

 over the area by the accumulation of sediment keeping pace with 

 depression until the old land surface had reached a depth of from 

 10,000 to 15,000 feet. 



Again the engines are reversed and all this sediment mostly 

 converted into solid rock comes up and is thrown into folds as were 

 the rocks of the old land from which it was derived. The mini- 



