4:32 TRANSACTIONS OF SECTION 0. 



diminish in number and in size. Boulders of local rocks, often obviously 

 transported and uplifted beyond their parent outcrops, become relatively more 

 abundant towards the limit of the foreign drift, and generally form a spread of 

 drift extending beyond it and passing insensibly into the driftless area. 



Great lakes vi^ere held up by the ice-barrier some time after it commenced 

 to retreat from the western slopes of the Pennines. During early stages in this 

 retreat the drainage from the lakes in and north of the Etiierow valley escaped 

 northwards, and ultimately passed through the Walsden gap into the Calder. 

 When the ice-barrier east of Manchester fell below 600 feet above O.D., this 

 drainage followed the course of that south of the Etherow valley and escaped 

 southwards. 



The action of the ice-sheet with its associated streams of water, together 

 with the marginal water derived from melting ice and draining from the region 

 beyond the ice-sheet, assisted by the action of post-glacial streams, in depositing 

 the original drift, in cutting new channels through rock and drift, and in 

 resorting and redepositing the debris, seems quite sufficient to account for the 

 complicated superficial deposits in this area. 



No evidence has been found of more than one period of glaeiation nor of 

 any local glacier system. There are, however, curious corrie- or cirque-like 

 features, e.g., on Shelf Moor, Glossop. Moreover, although the Pennines are 

 on the whole much lower north of the Etherow Basin than further south, the 

 overflow-channels of glacier-lakes can be found at higher altitudes in the former 

 than in the latter region. This is the reverse of what might be expected if 

 the higher ground were ice-free. It may be, therefore, that at and near the 

 time when the ice-sheet attained its maximum development, the snow-line 

 actually descended below the altitude of the higher Pennine hills, and, without 

 bringing about a definite local glaeiation, temporarily filled the higher hollows 

 with snow up to the general level of the ridge. Thus, instead of the margin of 

 the ice-sheet at that stage melting away rapidly, melting might be considerably 

 reduced and even temporarily suspended, and the ice-sheet reinforced by the 

 local snow-fall. Such conditions would tend to depress the limit of distribution 

 of erratics immediately west of the highest ground, but where an ice-stream 

 carrying erratics actually crossed the watershed, they might lead to the distri- 

 bution of those erratics further and more widely than otherwise might have 

 been possible. 



8. Discussion on Radio-active Problems in Geology. 



(a) Professor Sir E. Eutheefoed, F.R.S., opened the Discussion. 



[b) Contribution to the Discussion on Radio-active Evidence of the Age of 

 the Earth. By Aethue Holmes, D.I.C, B.Sc. 



The radio-active methods of measuring geological time have given result.s 

 which are consistent among themselves. The unavoidable discrepancies between 

 the periods based on helium-ratios and those calculated from corresponding 

 lead-ratios do not stand in need of reconciliation, and if they did the pleochroic- 

 halo method is there to bridge the gap. On account of the leakage of helium, 

 periods based on the accumulation of the latter are always smaller — invariably 

 less than half — than those implied by the accumulation of lead. Consequently, 

 the greatest age deduced from helium-ratios, 715,000,000 years for a sphene 

 from the Lower Pre-Cambrian rocks of Ontario, Canada, is not likely to be 

 more than half the true value. In keeping with this supposition, the greatest 

 age yet determined by the lead-ratio method amounts to 1,500,000,000 years, 

 this figure referring to zircons derived from the oldest granitic rocks of the 

 Pre-Cambrian platform of Mozambique. 



Supporting this result are the lead-ratios of the well-known radioactive 

 minerals of the Middle Pre-Cambrian of Norway and Sweden. The ratios fall 

 into two well-marked groups, which indicate ages of 1,000,000,000 and 

 1,200,000,000 years respectively. The lower figure is of special value, not only 

 because of the consistent testimony of eleven minerals, but also because lead 



