HAS THE EXTENT OF LAND SURFACE BEEN INCREASING] 207 



earth's form to have taken place, the constant tendency of those geological 

 agencies which we know must have been immediately set to work would be 

 toward increasing the existing areas of land, by carrying the del^ris of the 

 higher portions into tlie adjacent regions of the ocean. This statement is, 

 however, based on the reasonable assumjition that the ocean shore would 

 not be extremely precipitous. Where it was so, the increased area of land 

 created oat of the detrital material brought from the adjacent higher regions 

 would form but a narrow strip, because so great a depth would have to be 

 filled up before the new land formed would begin to emerge from beneath 

 the surface. 



In point of fact, a large portion of the stratified masses from wliich the 

 continents have been built up does consist of detritus abraded from adjacent 

 higher land areas and carried to the sea and then laid down on its lloor. 

 The coarser this material was, the nearer to the shore it became deposited, 

 and the more rapidly the formation increased in thickness. Far from the 

 land the accumulation would go on very slowly, since only the very finest 

 detritus could be carried to such a distance ; and although the material 

 secreted from the ocean water by organic agencies would be added to the 

 detrital deposits thus formed, after the death of the animals or plants by 

 which the solid material was thus eliminated, the increase made in this way 

 would also be very slow. Thus the body of strata representing any particu- 

 lar group is likely to be much thinner when its formation has taken place 

 in deep water than it is where deposited in close proximity to the land. 

 This is well illustrated by the relative thickness of the difierent groups of 

 the Palteozoic series, as exhibited along the line of the Appalachian Range 

 and in the Mississippi Valley. 



The continental masses may then be looked upon as Ijeing mainly built 

 up of the eruptive and azoic nuclei and abraded material derived from them. 

 The former constituted a part of the original crust of the earth raised above 

 the other adjacent portions ; the latter, the result of the action of the erosive 

 agencies upon the elevated portions. But, of course, where disturbances of 

 the crust have been continued in the same resrion, after stratified material 

 has been laid down adjacent to the central elevated masses, the detrital beds 

 have, in turn, been lifted up. often metamorphosed, and themselves subjected 

 to erosion, the result being an aggregation of stratified material either gath- 

 ered around a central nucleus of crystalline rock, or more often along the 

 flanks of an axially situated mass of a similar character, the original force 



