236 RECORD OF SCIENCE FOR 1887 AND 1888 



posed to look upon the tilting of the glaciated region as but an exem- 

 plification ot one of the fundamental laws of earth movement: Babbage, 

 Hall, Hunt, and Dana long ago, and Dutton, Fisher, Keade, Alexander 

 Winchell, and others within the last lustrum, have shown that the exter- 

 ior portions of the earth behave as if in a state of hydrostatic equilibrium, 

 ready to rise with the removal and sink with the addition of the matter 

 transferred by the processes of gradation. Now it is evident that an 

 ice sheet must depress the surface upon which it rests, just as does a 

 mass of oceanic sediments, directly by its weight, and also indirectly 

 by chilling and so condensing the underlying rocks; and since, as all 

 students of the primary agencies and conditions of geology are agreed, 

 the viscous mass of the earth responds slowly to stresses tending to pro- 

 duc(i deformation, it is equally evident that the resumption of original 

 attitude by any part of the surface after the recession of an ice sheet 

 must be gradual and perhaps exceedingly slow. So the southward tilt- 

 ing indicated by the shore lines of Lake Agassiz, by the half drowned 

 estuaries of Lake Michigan, by the beaches of the ancient Lake Ontario, 

 aiul by the terraces of the Connecticut River, all seem attributable to 

 the effort of the resilient terrestrial crust to return to its original form 

 on relief from the pressure of the Pleistocene ice sheet; and the diver- 

 sity in behavior of the north-flowiug and south-tlowing streams of New 

 England would indicate that the restoration is even yet barely com- 

 plete. 



The ideas current among the leading geologists of the country con- 

 cerning the behavior of the earth as an isostatic body when compressed 

 beneath a great continental glacier have been summarized by Alexander 

 Winchell within a few months.* 



But there are certain comparatively recent changes in level wiiich 

 can not be attributed to movements due to the weighting of the land 

 beneath the Pleistocene ice sheet. One of the more important contri- 

 butions of the biennial period to the general subject of deformation is 

 that by LeConte, on the recent changes of physical geography in Cali- 

 fornia indicated by the flora of the coast islands.t Sometime during 

 the Pleistocene there was a depression of the Pacific coast by which the 

 westernmost of the two ranges belonging to the trans-Sierra mountain 

 system was nearly submerged, only the commanding summits rising 

 above tide-level to form the islands of Santa Rosa and her companions; 

 and this drowned mountain range displays no <lisposition to return to 

 its former altitude. The period of this submergence is indicated, in so 

 far as plants may be regarded as chronometers of geologic time, by the 

 distintictive Pliocene flora of the islands, which has, according to Le 

 Conte, been preserved by reason of its isolation, while the flora of the 

 mainland has undergone modification in the struggle against competi- 

 tors, enemies, and climatic conditions i)roper to a great continent. 



*Ani. Geologist, 1888, vol. i, pp. 139-143. 



t Bull, of Cal. Acad. Sci., 1889, vol. ii, p. 575. 



