ON THE COAST OF MAINE. 339 



upon the land. A mile in depth of ice weighs about as much as half a mile of ordinary 

 rock, so that by covering the continent of North America with a deposit of this kind we 

 more than double its altitude above the sea. Now if the weight of the mass uplifted be an 

 element in determining the height to which the continents are raised, then we must allow 

 this ice mass a decided mfluence in depressing the continental areas. 



The reader may help himself to form a conception of the nature of these effects if he 

 will float a rigid substance, such as a bar of wood, upon some semifluid, such as tar or trea- 

 cle, weighting one end so that by its depression it may sink beneath the surface, thus ele- 

 vating the other end. Now if he will add w'eight to the elevated end, subtracting it, if he 

 please, from the other end, he will see the change in altitude which I conceive to have 

 occurred by the accumulation of glacial matter. Another illustration, more satisfactory on 

 the whole, is afforded by a little study of any of our frozen lakes. Let a weight be accum- 

 ulated at any point on ice of moderate strength. The result is the depression of the ice at 

 that point, and its elevation around the sunken point. In this condition the ridge and 

 depression may well represent the state of the tensions which give its continent and sea 

 basin. Now if we take a part of the weight from the depression and place it on the ele- 

 vated part of the ice, we will thereby change the altitude of the elevation. If the hollow 

 of the ice were filled with water this change would bring about an extension of the area 

 covered with water, representing the invasion of the land by the sea during the glacial 

 period. The fact that in a substance as rigid as ice these movements can take place with- 

 out fracture, shows vis that there is nothing in the solidity of the earth to prevent such 

 movements. 



It must not be thought that this view requires us to suppose the interior of the earth is 

 a fluid mass, although it is quite reconcilable with that theory. I am inclined to cling to an 

 opinion set forth by me some years ago that the central region of the earth is quite rigid, 

 while between the outer fifty miles of strata and basement rocks, and the inner core of 

 solid matter, there may remain a section which has not yet been completely solidified, and 

 still admits of sufficient movement to give us the rise of the continental folds and the sink- 

 ing of the sea floors. Nor am I satisfied that the condition of the mass on which the so- 

 called crust rests, is such as can be called fluid, judged by the tests we apply to objects on 

 the earth's surface. A rigidity such as belongs to the metals of average resistance to com- 

 pression might permit such movements as we find to occur in masses urged by the enormous 

 tensions to which our continents are subjected. 



There can be hardly any question that the conclusion that the continents are kept in 

 their attitude by constant tensions, is necessary and indisputable. As a factor in determin- 

 ing the position which any point occupies with reference to the centre of the earth, the 

 weight of the mass supported must be of importance. Unfoi-tunately the data for determ- 

 ining the value of this element are quite wanting ; it seems evident enough, however, that 

 we may more reasonably look to the weight of the ice accumulated on the continents dur- 

 ing the glacial period for the depression of the land areas it occupied, than to any other 

 cause. Only in this way can we account for the local character of the depression in many 

 places, or for the coexistence of extensive subsidences in the southern as well as the north- 

 ern hemisphere. 



