GEOLOGY. 237 



tion among chemists. A description of this filterer will be 

 found in this volume (see Index). 



CHANGES OF LEVEL OF SEASHORES. 



Numerous observations have been made upon the changes of 

 level of shore lines now in progress on the borders of the differ- 

 ent continents, but as yet little effort has been made to determine 

 the nature of the movements involved in these chanofes. 



Inasmuch as we have no other means of readily perceiving the 

 changes of level of the earth's crust except such as may be af- 

 forded by the consequent alterations in the position of the line of 

 contact of sea and land, our observations are limited to the shore 

 lines. But unless we form some idea of the w^j in which the con- 

 tinents and sea floors are affected during these movements, we 

 cannot fully understand the phenomena. It needs no argument 

 to show that it is exceedingly unlikely that these movements are 

 mere accidents of the shore. It is evident that they must be a 

 part of an extensive movement, which is only rendered evident at 

 a few points by changes in the position of the sea. The only way 

 in which any considerable portion of the earth's crust can change 

 its level is by a sini^ing of the ocean furrow and a lifting of the 

 adjoining continental folds. As the earth must be essentially in- 

 compressible to any pressure which any part of the outer surface 

 couUI suddenly apply to it, we can only conceive of subsidence 

 over an extensive area by supposing a proportionate elevation at 

 another point. This idea is thoroughly borne out by all we know 

 of the geological history of the earth's crust ; for we have as the 

 main feature of that history the continued elevation of continents 

 attended by the continual depression of the ocean trougiis. That 

 the continents have been continually elevated is sufficiently 

 proved by the fact that they are still in existence, although large 

 portions of them have been from the earliest time subjected to 

 the erosion of atmospheric agents. Over a large portion of the 

 basin of the Mississippi the erosion is as rapid as a foot a thou- 

 sand years ; yet this region has been above the level of the sea for 

 millions of years. The evidence of the continued subsidence of 

 ocean floors is also easily seen. Now a movement wherein the 

 lands go up and the seas down is comparable to that of a 

 lever, or Ijar, about a fulcrum point. The whole section from 

 the centre of the sea to the centre of the continent is not like a 

 rigid bar, in which case the centre of either land or water area 

 would have to describe a great curve in order to have any con- 

 sideraJ^le motion near the shores. The sea floors probably sink and 

 the land areas rise at somethino: like the same rate over the greater 

 part of their surfaces. Still, for convenience' sake, we may regard 

 the section of the earth's crust for a little distance on either side 

 of the shore line as moving as a rigid mass as the uplift of the 

 continents and the sinking of the sea bottom goes on. 



It is evident that nuK-li will depend upon the position of the ful- 

 crum point of the movement in relation to the shore line ; if this 



