CLIMATIC TYPES 921 



doubtless including less than 5 per cent of the continental deposits. Here 

 again such sediments as are made are chiefly fluviatile. Owing to the 

 effectiveness of solution and chemical decay over the hills from which the 

 streams derive their sediment, the minerals of the eventual deposits on 

 the lowlands are greatly simplified. The ferruginous clays, rich in hv- 

 droxides and hydrous silicates, overwhelmingly predominate. Being de- 

 posited on the moist bottom lands, these sediments become richly charged 

 with organic matter and are generally saturated with water. Therefore, 

 in spite of the fact that they are usually derived from red later itic soils, 

 they tend to become partly deoxidized and either decolorized or blackenerl 

 by the excess of organic matter. As in the sub- Arctic family of sediments, 

 peaty deposits and soil flows are among their associates ; but both of these 

 differ in actual composition from the peaty and solifluctional deposits of 

 high latitudes. They also resemble the sub- Arctic group in being devoid 

 of saline and eolian deposits. Unlike that group, however, they of course 

 never contain interbedded till sheets, and the minor deposits of mud and 

 biochemical residues made in lakes and marshes are probably quite differ- 

 ent, although as yet but poorly known in detail. 



Desert type. — The desert family of sediments, forming the third corner 

 of our triangle, is very distinct from the others. Deserts are said to cover 

 20 per cent of the land surface, and although over a large ])art of that area 

 the rock is bare rather than deeply covered with sediments, it is neverthe- 

 less almost certain that the quantity of desert deposits is much greater 

 than that of the humid, tropical type. Even in deserts the deposits made 

 by streams, evanescent though they are, probably exceed those of all other 

 agencies, and this in spite of the fact that the wind is doubtless the most 

 important erosive agency there at work, for in contrast with the streams 

 the wind exports most of its product. The transient streams of the desert 

 make two rather distinct types of deposits, which may be called (as sug- 

 gested by Tolman) the ^^ajada" and the "playa^^ types. The materials of 

 the '^ajada" or piedmont slope deposits are, as a rule, both poorly and 

 variably stratifled. The minerals are but little decayed and therefore con- 

 sist largely of unaltered silicates. Although in this respect they resemble 

 the sediments of the sub-Arctic group, they differ radically from the latter 

 in being practically devoid of organic matter. Their colors are therefore 

 generally pale, such tints as buff and light gray predominating. There 

 is every textural variation among bajada deposits, from coarse boulders 

 down to silts, and generally particles of many sizes are closely inter- 

 mingled. This well known imperfection of assortment is due to the hasty 

 but brief action of the desert thunder-storms. Often tlie transient stream 

 is a river of mud, in which particles have but little freedom of movement. 



The deposits made in the playas or temporary lakes are brought to 



