CLIMATE AND TERRESTRIAL DEPOSITS 285 



THE CLIMATIC SIGNIFICANCE OF COLOR 



One of the most obvious features of the sedimentary rocks is their 

 color, due partly to climatic, partly to various other, conditions of 

 origin. Among the latter Walther calls attention to the fact that — 



The colors of continental deposits are different according as they are formed 

 above or below a water surface. The deposits in the courses of streams or in 

 interior seas have usually either greenish or bluish colors which also characterize 

 the marine deposits of the continental shelves. The typical continental deposits, 

 formed upon the dry land, are characterized by bright clean colors. The car- 

 mine or vermilion tropical laterite, the red-colored sand dunes of the Coromandel 

 lowland and the inner Arabian desert, the yellow or brown loam and loess deposits 

 of the steppes, the white or yellow dunes of the coast lands or the Sahara are 

 convincing examples.^ 



The influence of climate as distinct from other conditions of origin 

 is rendered evident on contrasting the brilliant reds of the moist 

 tropical or subtropical regions with the yellow or dark soils of colder 

 temperate climates or the prevailing ashen gray of deserts. The 

 significance of color in the older rocks cannot, however, be directly 

 inferred from the study of modern deposits since the changes which 

 transform a soft sediment into a solid rock may also conceivably alter 

 the color. 



For the purposes of the present discussion, the colors of shales and 

 sandstones may be grouped under three heads: first, red; second, 

 light and variegated; third, gray to black. Each of these color 

 groups is of importance and includes appreciable portions of the sedi- 

 mentary rocks. On account of the diversity of views, however, respect- 

 ing the significance of red and its climatic bearings, it will be necessary 

 greatly to enlarge the discussion upon that topic. 



The origin of red formations. — Red shading into brown is one of 

 the most frequent colors of ancient continental shales and sandstones 

 and may also occur among those of marine origin. That there is no 

 unanimity in regard to its significance among geologists may, however, 

 be gathered from an examination of the literature. Russell has 

 given a full review of these views. ^ Certain authors have supposed 



1 Einleitung in die Geologie, translated from Lithogenesis der Gegenwart, 1893-94, 

 P- 725- 



2 "Subaerial Decay of Rocks and Origin of the Red Color of Certain Formations,'' 

 Bulletin 52, U. S. Geological Survey, 1899, pp. 47-56. 



