vx- 



DETERMINING FACTORS 919 



Altlioiigli of foremost importance locally in snch deposits as landslides, 

 the nature of the parent rock is but a minor factor in influencing the 

 great bulk of continental sediments. The voluminous river deposits of 

 diastrophic basins, and deltas are generally derived from so large a variety 

 of rocks that the materials are averaged. Nevertheless, lithology at the- 

 source becomes a factor of large consequence even in the basins of some 

 large rivers, such as the Ilwang-ho of China and our own Missouri, which 

 on flowing through hundreds of miles of almost continuous loess de- 

 posits become charged mth that material to the exclusion of most others. 



The processes of weathering at the point of origin of the debris deter- 

 mines in large measure the kinds of minerals from among which the 

 active agencies, such as streams, wind, and glaciers, may select material 

 for their ultimate deposits. If solution and chemical decay are dominant, 

 ferruginous sandy clays of rather simple mineral composition will nor- 

 mally result ; but if physical disintegration prevails, then arkoses, wackes. 

 and complex silts are likely to be formed. 



The processes of transportation and deposition determine very largely 

 the structure, topography, and texture of the deposit, as well as the shapes 

 of the individual particles. They also assort and segregate the material 

 into sediments of unlike mineral composition. Many continental elastics, 

 such as moraine-stuff, dune sand, and loess, may be said to owe their most 

 distinctive attributes to these transportive processes. Others are influ- 

 enced less strongly, and some, like gypsum or peat, very little. 



The conditions at the point of final deposition, being the last to affect 

 the sediment, often exert the dominant influence in determining its char- 

 acter. A familiar example is furnished by the deoxidation of the ferric 

 material derived from the bright red soils of the equatorial rain-belt, 

 whereby, under the moist jungle of the river flats, it is converted_into a 

 dark gray or black mud. 



On reviewing the four factors just mentioned, we find that the last 

 three are controlled in large measure by the climate and subordinately by 

 the topography. The topography exerts an important influence on the 

 thickness, the situation, and the area of deposit, and frequently on its tex- 

 ture. But it is climate that not only shares these influences, but controls 

 most of the other characteristics of a sedimentary deposit, for climate is 

 the chief factor in determining the processes that shall be at work on the 

 surface, their mutual relative values, and hence the composite result. To 

 many students of the subject, therefore, a classification of the continental 

 sediments based not on processes, but on the more fundamental factor of 

 climate, modified by topography, seems the most serviceable. I have 

 adopted that plan for this occasion. 



LXVIII — Bull. Geol. Soc. Am., Vol. 28, 1916 



