ELISHA MlTCHElvL SCIENTIFIC SOCIETY. 3o 



the more gfeneral evidence of sedimentary deposits, on 

 account of the g-reater ease with which the former are 

 effaced. The burden of the evidence then lies in the 

 sedimentary deposits of the x\tlantic slope, or, more 

 accurately, of that part of North America whose sedi- 

 nientar}^ deposits can not be referred to some other 

 ocern or water body. 



To decide what should be taken as evidence of prox- 

 imity to shore line we must be familiar with the prin- 

 ciples gfoverninor- the deposition of sediment. Running 

 water is the principal carrier of frag-mental materials 

 such as g*o to make up secondary deposits. These ma- 

 terials vary greatly in coarseness and in composition. 

 Far the greater part, however, is mineral matter, re- 

 sulting from the decay and disintegration of the rocks 

 of the land, and the fragments which compose it vary 

 from an impalpable powder to the greatest size which 

 can be swept along by water in motion — this latter de- 

 pending upon the velocity and volume of the current. 

 It is b}^ means of this principle of the carrying power of 

 water that we explain the sorting of those fragments 

 which find their way into moving water. A current of 

 high velocity will transport comparative! v large pieces 

 of rock material, until, by decrease of slope or by en- 

 trance into some other body of water, its velocity is 

 lessened; then the materials will be deposited, the coars- 

 est first and others in turn as the stream continues to 

 lose velocity. The finest may be deposited a great way 

 out in the ocean, sea or lake, which receives the trans- 

 porting current. 



Since the ocean serves as a receptacle for all the 

 drainage of the land, there is being deposited, within 

 its minor depths and out to a distance of perhaps a 

 hundred miles from the land, all the solid materials 



