GEOLOGICAL IMPORTANCE OF SEDIMENTATION 347 



of life are not present throughout ? Here again various factors enter 

 external to the present subject, which cannot be discussed without 

 too much digression. It may be said in closing, that the absence of 

 fossils leaves the origin of the formation in doubt, and that an unfos- 

 siliferous formation may have originated either on the land or beneath 

 the sea. Certain structural criteria which may frequently throw 

 light upon this question will be discussed in a following part. 



SUBMARINE TOPSET DELTA DEPOSITS 



In connection with the discussion upon the subaerial part of the 

 delta, some mention has necessarily been made of the submerged 

 part of the same, but more especially the steeply inchned foreset beds. 

 Under the present head it is intended to discuss particularly the shal- 

 low, submerged portion of the delta in some detail, in order to show 

 what relations it holds under various conditions throughout the world 

 to the emerged portions of the same. 



Relation 0} Waves and Nature 0} Deposit. — The proportion of 

 the topset delta surface above and below water will depend upon 

 several factors. Two of the more prominent are the rapidity of the 



Scots in hilomehers. 



Verhjcal Scale mulfiptred b^ 100. 



Fig. 3.— Profile. Delta of the Rhone in Lake Geneva. 



deposit and its coarseness on the one hand, and the power of the 

 waves on the other. The maximum of land topset beds may there- 

 fore be seen in such instances as Lakes Geneva and Constance, but 

 especially the former, where the gritty sediment of rapid and loaded 

 streams is confined closely between valley walls and discharged into 

 the relatively quiet waters of a lake. The fact that practically all the 

 upper surface of the delta is a land surface in the case of Lake Geneva 



