1893.] evidence for the Recurrence of Ice Ages. 225 



our own, and also, according to some, during the deposition of the 

 Lower Carboniferous rocks. 



Measures of Earth movement. 



The study of the sedimentary rocks brings out most clearly 

 that there have been oscillations of level over large areas — that 

 after a long period of depression and continuous deposition there 

 has always come a reversal of the direction of movement and a 

 continuous upHft for another long period. 



The first question is — what is the known extent of such 

 movements in the districts under consideration ? A minimum 

 estimate can be obtained by measuring the edges of the deposits 

 that have been continuously depressed to admit of the accumu- 

 lation of more sediment on top of them, and as these formations 

 can be observed only when afterwards folded up into plateaux 

 and mountains in the second period of their history we have 

 in them a measure of the amount of uplift. It does not follow 

 that there ever existed an ocean as deep as would be implied by 

 the depth to which the first-laid sediment descended, because 

 the filling up of the abyss would go on during the period 

 of subsidence. Nor should we be justified in assuming that at 

 any time there was land as high as the folds of the earth's crust 

 would seem to have carried the strata, because the waste of the 

 high lands would be going on during the whole of the period of 

 upheaval. But the quantities we are dealing with are so vast that 

 there is plenty of margin and a small excess of elevation over 

 denudation would give us mountain regions high enough to account 

 for all the phenomena we seek to explain. 



Eastern side of Atlantic basin. 



First we will examine the evidence as to the amount of earth 

 movements certainly known to have taken place in our own 

 country. How much material was removed from the Archaean 

 Rocks, before the great Cambrian depression submerged them all, 

 cannot be accurately ascertained owing to the absence of suffi- 

 ciently well-marked horizons in that ancient crystalline series. 

 But the amount of the depression which went on during the 

 deposition of the Cambrian System cannot be less than the total 

 thickness of the Cambrian Strata, and, as in some areas there is 

 no great discordancy between the Cambrian and Silurian, such as 

 would imply any considerable reversal of the movement, we may 

 estimate the depression as represented by the thickness of the 

 Cambrian and Silurian Systems together \ 



Now taking the thicknesses as deduced from the Geological 



1 Camb. Phil. Soc, Vol. iii. p. 247. 



