98 GEOLOGICAL HISTORY OF LAKE LAHONTAIn. 



transported to their mouths is fine silt, and in their deltas the divisions de- 

 scribed above are obscure and indeterminate. 



A fluctuation of lake level during the formation of deltas produces 

 even greater modifications in their forms and structure than the same change 

 of conditions would in the nature of current-formed embankments.. When 

 the waters of a lake rise and submerge a delta, a new one is at once com 

 menced and carried forward in the same manner as the first, which it may 

 eventually bury so completely that only a deep section would reveal its 

 presence. In some ancient deltas that have been dissected by erosion a 

 stratum of lacustral sediments is found separating two delta deposits. In 

 such an instance the included sheet of fine material is thickest near the 

 outer margin of the structure; the formation of the higher delta and the 

 deposition of the fine sediments took place at the same time, the latter being 

 finally overplaced by the former. The lowering of a lake causes its tribu- 

 tary streams to erode channels through their previously formed deltas and 

 to commence the building of new ones, either in the gap thus formed, or 

 altogether below the former structure. In some instances in the Bonne- 

 ville and the Mono basins this action was carried forward at a number of 

 successive stages until a series of small deltas were formed, each starting in 

 the channel cut through its predecessor In the formation of deltas, as in 

 the construction of terraces and embankments, one of the most important 

 conditions requisite for the production of typical examples on a large scale 

 is permanence of lake level. The finest deltas are formed in lakes that 

 maintain a constant horizon for a considerable time and receive the influx 

 of high-grade streams which are abundantly loaded with debris. 



In the above sketch attention has only been given to the modifications 

 of lake shores and tideless seas ; the action of the waves, currents and tides 

 of the ocean receiving no consideration, because they are foreign to the 

 scope of the present essay. 



BEC APTT UI> ATION. 



Cut terraces are shelves carved in the shores of lakes by the action of 

 waves and currents; they are bounded on both their shoreward and lake- 

 ward margins by steeper slopes; the former inclines upward and forms a 



