JProf. Bonney — The Felsite of Bittadon, N. Devon. 207 



be correct — we could tell the length of time which has elapsed since 

 that period, and should thus possess a standard to which we could 

 refer other geological periods. 



Within the last hundred thousand years there have been two 

 periods of great excentricity. The former of these began 980,000 

 years ago, and terminated 720,000 years ago : the latter began some 

 250,000 years ago, and ended about 80,000 years ago. Sir Charles 

 Lyell at one time referred the Grlacial epoch to the former period, 

 but subsequently agreed with Mr. Croll in assigning 250,000 years 

 ago as the more probable date of the commencement of the Great 

 Ice Age. According to this view, Mr, Croll calculates that sixty 

 millions of years have elapsed since the beginning of the Cambrian 

 period, whereas on the hypothesis that the Great Ice Age occurred 

 during the former period of excentricity, the beginning of the 

 Cambrian period will have to be placed, on the same method of 

 reckoning, two hundred and forty millions of years back. But 

 this, both the results of physical inquiry, and the known facts of 

 denudation, declare to be improbable or impossible, while that a 

 period of eighty thousand years should have elapsed since the 

 Glacial epoch accords well with the amount of denudation which 

 the Glacial deposits are found to have undergone. On this subject 

 Mr. Croll writes as follows : " Now if we assign the Glacial epoch 

 to that period of high excentricity beginning 980,000 years ago, and 

 terminating 720,000 years ago, then we must conclude that as much 

 as 120 feet must have been denuded off the face of the country since 

 the close of the Glacial epoch. But if as much as this had been 

 carried down by our rivers into the sea, hardly a patch of Boulder- 

 clay, or any trace of the Glacial epoch, should be now remaining on 

 the land. It is therefore evident that the Glacial epoch cannot be 

 assigned to that remote period, but ought to be referred to the period 

 terminating about 80,000 years ago. We have, in this latter case, 

 thirteen feet, equal to about eighteen feet of drift, as the amount 

 removed from the general surface of the country since the Glacial 

 epoch. This amount harmonizes very well with the direct evidence 

 of geology on this point. Had the amount of denudation since the 

 close of the Glacial epoch been much greater than this, the drift 

 deposits would not only have been far less complete, but the general 

 appearance and outline of the surface of all glaciated countries would 

 have been very different from what they really are." 



It is not without great diffidence that I have written this article. 

 In it I have rather aimed at giving a general resume of a great 

 subject, than attempted to add anything very new. Such as it is, I 

 lay it before the readers of the Geological Magazine. 



III. — Note on the Felsite of Bittadon, N. Devon. 

 By Prof. T. G. Bonney, M.A., F.G.S. 



IN his important paper on the Physical Structure of West Somerset 

 and North Devon (Quart. Journ. Geol. Soc, vol. xxiii. p. 568), 

 Mr. Etheridge incidentally refers to a felstone at Bittadon, saying 

 of it (p. 609) : " There is no proof that the igneous rook at Bittadon 



