338 RECENT CHANGES OF LEVEL 



is much in favor of this theory of the changes of sea and land. For onr present purpose 

 it is not necessary to argue this question in all its phases, it is enough that it enables us to 

 coordinate the changes of tiie sea and land, and so fiir to aid us in the conception of the 

 instability of the equilibrium of the crust which is one of the most important facts brought 

 to light by the study of the geological record. The constant changes in the position of the 

 fulcrum point of the movement of sea and laud, show that the position of this point, with 

 reference to the whole line, is a matter liable to constant change. In the determination of 

 the position of this fulcrum Ave may be reasonal)ly certain that whatever the source of the 

 force which brings aljout the upheaval and subsidence, the matter of weight must have a 

 certain value. Physicists who have attentively considered the question of the origin of the 

 continental folds and ocean furrows, are quite in accord in considering them as folds of the 

 earth's crust, or, to keep clear of assumptions, let us say the outermost part of the earth, which 

 has been compelled to adapt itself to a dimini.'^hed interior. Without pretending to claim this 

 conclusion to be so well founded as to put an end to controversy, we may use it to complete 

 our conception of the relation of forces to the section from the interior of the continent to the 

 centre of the sea. It will be evident that this section must have its present place determ- 

 ined by two main factors, the energy of the uplifting force and that of the restraining 

 weight. If the amount of this weight at particular points can be made to vary considera- 

 bly, the whole effect of the movement may be materially changed. A careful consideration 

 of the accompanying diagram will show that a change in the position of the point of rota- 

 tion or fulcrum of the section will completely alter the result of the movement. When 

 the pivot point is just at the shore line, a great deal of sinking of the sea floor and eleva- 

 tion of the centre of the continent may take place without affecting the relations of sea 

 and land. When the pivot point is to the land side of the shore, then the movement will 

 make the sea appear to gain on the land ; when the pivot is to the seaward of the shore, 

 then the land will gain. Elsewhere I have followed these considerations in a more detailed 

 fashion with special reference to the question of the origin of continents. The conclusion 

 is. however, so evident that it is hardly necessary to trace the whole matter here. 



Fig. 6. 



The dotted line, a, a, a, a, indicates the change of position of tlie surface after the ice caps have been formed, 

 c, Continents, i, ice sheet. *•, sea level. 



It is very important to notice, however, that in the case of a rigid mass, such as we sup- 

 pose the crust to be, supported on material having sufficient mobility to give way vuider 

 strain, much as a fluid would do, then the imposition of any weight upon one extremity of 

 a given section, such as is shown in Fig. 6, would necessarily produce a change in the posi- 

 tion of the pivot point. Now in the ice accumulation of the glacial period we have just 

 such a change of weight as would be likely to bring about considerable effects of this kind. 

 A great mass of water is taken from the sea and heaped to the depth of a mile or more 



