CLIMATE AND TERRESTRIAL DEPOSITS 165 



this region is still above the sea, observation as to the kind of rocks 

 now outcropping and probably present at that time; second, by a 

 detailed study of the nature of the sedimentary particles (Sorby, 

 Bonney, and others have shown to what an extent microscopic study 

 of sedimentary materials may indicate the lithologic nature of the 

 source) ;'^ third, by comparing the particles in deposits of dissimilar 

 nature but of similar coarseness or fineness accumulated in suc- 

 cessive epochs in the same region, and presumably from the same 

 general source. For example, the material forming the coal-measure 

 shales and sandstones of eastern Pennsylvania is of about the same 

 grain and doubtless from the same ultimate source as the Upper 

 Devonian and Subcarboniferous shales and sandstones of the same 

 region. Scattered microscopic particles of feldspar in one of these 

 formations and an absence of them in another may be taken as due 

 to a difference in topographic or climatic conditions of origin in the 

 region of the headwaters, and not due to derivation from unlike 

 lithologic sources. 



The preceding statements regarding the influence of the sedi- 

 mentary source must not be allowed, however, to give an exaggerated 

 importance to the lithologic character of the original rock; important 

 near the regions of erosion, it diminishes with the distance of trans- 

 portation. In regard to this matter E. W. Hilgard states : 



Alluvial soils are to a certain extent dependent upon the character of the 

 rocks and surface deposits occurring within the drainage area of the depositing 

 stream. As a rule their composition is much more generalized; and their char- 

 acter as to the relative proportions of sand and clay is essentially dependent 

 upon the velocity of the water current.^ 



This diminution in influence of origin with distance of trans- 

 portation is due partly to the contributions from tributary streams 

 coming from somewhat different geologic provinces, partly to con- 

 tinued decomposition of the sediments during transportation, tending 

 to leave similar insoluble residues, but largely to the sorting action of 

 the water in separating unlike constituents. The sediments from 

 dissimilar geologic provinces, therefore, are apt to consist of similarly 

 classified products, such as ferruginous clays and siliceous sands, 



1 T. G. Bonney, Presidential Address, Section C — Geology, Proceedings of the 

 British Association for the Advancement of Science, 1886, pp. 601-21. 



2 Soils in the Humid and Arid Regions, 1906, p. 13. 



