304 R. A. DALY OSCILLATIONS OF LEVEL 



The sinking under load is in small part due to purely elastic compres- 

 sion, a shortening of the earth's radii below the ice-cap, effected without 

 lateral flow of material. In part the sinking is referable to subsurface 

 lateral flow, to what may be conveniently called plastic deformation of 

 the globe. Since the viscosity of the subcrustal material is everywhere 

 high in absolute measure, deformation of the loaded surface by subsurface 

 flow entails an upward bulge of the country just outside the sinking area. 

 The evaluation of such a marginal bulge and of the corresponding mar- 

 ginal sinking after deglaciation would help to show the relative im- 

 portance of the two modes of crustal yielding. As Barrell has noted, the 

 relative size of each marginal bulge would also give some idea concerning 

 the degree of resistance to subsurface flow. For brevity these marginal 

 movements will be referred to as isostatic, although of course the up- 

 warped areas are not isostatically adjusted. ' 



The full, quantitative solution of the problem must be long postponed. 

 Curiously enough, its attack on the qualitative side has been seldom made 

 or even mentioned in the copious literature dealing with Recent uplifts 

 in deglaciated regions. 



The delay is the more remarkable in view of Jamieson's clear, though 

 brief, statement of the principle. He seems to have been the first to 

 announce the hypothesis of depression under ice-load and elevation conse- 

 quent on ice-melting. Seventeen years later he wrote : 



"It seems likely that there might be not only a slight sinking of the ice- 

 covered tract, but likewise a tendency to bulge up in the region which lay 

 immediately beyond this area of depression, just as we sometimes see in the 

 advance of a railway embankment, which not only depresses the soil beneath 

 it, but also causes the ground to swell up farther off." "^ 



Glacial loading and unloading represent Nature's gigantic experiments, 

 perhaps the most useful ever made, to test the isostatic theory quantita- 

 tively. The present paper is offered as a contribution to those queries 

 and records which are preliminary to the solution of the quantitative 

 problem. Significant as the problem is in itself, its solution may turn 

 out to be of principal aid in working out the paleogeography and recent 

 history of wide belts of the earth's surface. 



Field-work on the New England coast, following the season of the 

 year 1900, spent on the shores of Labrador and Newfoundland, led the 



2 T. F. Jamieson : Geo!. Mag., vol. 21, 1865, p. 178 ; vol. 9, 1882, p. 461. 



Cf. J. Barrell : Am. Jour. Sci., vol. 40, 1915, p. 13. 



W. B. Wright, in The Quaternary Ice Age, London, 1914, p. 406, has given an excel- 

 lent survey of the subject so far as it concerns the areas uplifted after ice-melting, and 

 puts welcome stress on shifts of ocean-level because of glaciation and deglaciation ; but, 

 like most other writers, does not discuss crustal movements outside the isobases for zero. 



