360 G. F. Wright — Unity of the Glacial Epoch. 



and unloading of the strata which took place during the pro- 

 gress of the glacial age, it is not a wholly unwarranted step in 

 reasoning to connect the two as cause and effect. The sup- 

 position, therefore, of a marked subsidence of the northern 

 part of the continent contemporaneous with the climax of 

 the period is a fair one to use in explanation of the complex 

 phenomena of the time. 



To appreciate the degree of probability attending the ex- 

 planation, however, it is important to consider still more 

 closely the natural operation of the forces at work. Upon 

 doing this it is evident that glacial conditions would continue 

 some time after the land subsidence had begun. For a long 

 period the accumulation of ice might be faster than the sub- 

 sidence of the plateau on which it rested. The anticlimax, 

 when the melting began to proceed faster than the accumula- 

 tion, was doubtless reached only after a very prolonged period. 

 But when the tide of affairs had really turned and the land 

 had begun to rise the melting of the ice had also progressed 

 to a great extent and the final ending up of the period and 

 the return of the land to a higher level doubtless proceeded 

 with great relative rapidity. On this provisional hypothesis 

 most of the facts indicating slack drainage during the deposi- 

 tion of the marginal deposits will be as readily explained as 

 on that of two distinct epochs with different attitudes of land 

 level during their existence. 



6. The occurrence of vegetal and ferruginous accumulations 

 between successive strata of glacial deposits, has long attracted 

 w T ide attention. At first these were taken to indicate inter- 

 glacial periods of wide extent and of long separation in time. 

 The occurrence of such deposits seems to be still the chief 

 reliance of many European geologists, as well of some in this 

 country, for belief in successive glacial periods. Professor 

 Chamberlin, however, is not among those who place undue 

 reliance upon this class of evidence, as the quotation from him 

 already given shows ; for in his judgment very many of the 

 vegetal deposits occurring in the till " are referable to several 

 distinct horizons." The hazard of inferring a prolonged inter- 

 glacial epoch from such deposits, also very forcibly appears in 

 view of some recent facts brought to light by study of the 

 glaciers of Alaska. 



In this Journal for January, 1887 (pp. 1 1 to 15, more fully 

 stated in The Ice Age of N. A., pp. 51-62), I have given the 

 facts concerning certain buried forests at the mouth of the 

 Muir Glacier which are standing in undisturbed strata of sand 

 and gravel, from which the ice is slowly withdrawing. I 

 adduced evidence also to show that the ice front had with- 

 drawn several miles during the present century, uncovering 



