LARGER RELATIONS OF DELTAS 381 



mentation, tleduction from known principles of action is needec] to 

 supplement the conclusions from partial observations. 



Part I. — Larger Relations of T)i:r/i.\s 



DEFINITIONS OF A DELTA AND ITS I'Ah'TS 



Essential features of a delta. — A delta may be defined as a deposit 

 partly subaerial built by a river into or against a body of permanent 

 water. The outer and lower parts are necessarily constructed below the 

 water level, but its upper and inner surface must be land maintained or 

 reclaimed by river building from the sea. A delta, therefore, consists of 

 a combination of terrestrial and marine, or at least lacustrine strata, and 

 differs from other modes of sedimentation in this respect. T(s ])lac(' in a 

 classification of modes of origin may be given as follows : 



All sedimentation is continental, littoral, or marine. Continental 

 sedimentation may be in turn subdivided into terrestrial, paludal, and 

 lacustrine; terrestrial formations comprising deposits made by wind, 

 rain wash, rivers, or land ice; lacustrine including fresh-water and salt- 

 water lakes. Paludal sediments form a transitional type between teiTcs- 

 trial and lacustrine, but in the playas of desert basins and the swamps 

 of deltas they are quite distinct in character and in life content also 

 from either the more or less permanent lakes on the one hand, or, on 

 the other, the river plains which are only temporarily flooded. Marine 

 sediments are either of terrigenous or pelagic origin. It is with the 

 division of terrigenous sediments — sediments directly born of the land 

 and restricted to within a few hundred miles of shore — with which we 

 have here to deal. They are deposited over a wide range of conditions, 

 in the open sea or in partly landlocked bodies of water, at abyssal depths 

 and on shallow bottoms lighted by the sun, agitated by the waves. In 

 bays and estuaries gradations toward fresh-water conditions may occnr, 

 but the littoral, that zone alternately covered and laid bai'e by tides, 

 separates the waters of the oceans from those of the lands, '^rhe distinc- 

 tion between the waters is drawn geologically by means of the faunas, 

 but from the structural standpoint no such line can be drawn between 

 the deposits of lakes and those of shallow seas. Sedimentation with its 

 record of earth history may take place under all these environments, but 

 delta deposits alone cross the shore. The presence of a strand zone 

 within the formation is the distinctive feature, and in this paper, there- 

 fore, delta deposits will be spoken of as terrestrial and subaqueous, and 

 the conditions of deltas built into continental waters will not be discrimi- 

 nated from those built into the marginal waters of the oceans. The 



