922 E. BLACKWELDER CHARACTERISTICS OF CHEMICAL DEPOSITS 



them largely by the streams. They are fine of texture and evenly strati- 

 fied, save where the delta structure appears. They are likely to be im- 

 pregnated with crystals and interstratified with beds of chemically de- 

 posited salts. Like the bajada deposits with which they intergrade, they 

 are usually pale in color. This is perhaps the most favorable of all situ- 

 ations for the development of shrinkage cracks and for their preservation. 

 With the playa, and even with the bajada, type of desert deposits we find 

 associated a variable amount of dune sands— the purest, cleanest, and 

 usually the best rounded of all sands. In some regions they are almost 

 the sole deposits of the desert. 



MIXED TYPES 



Having considered the three extreme climatic types, we may now re- 

 view the more distinctive mixed varieties. 



In those lands which are visited in the course of the year by two dis- 

 tinct climates there is a corresponding effect on the sediments deposited. 

 These conditions are found best developed in regions affected by the mon- 

 soons and the belt a few degrees wide along the equator. The alluvium 

 of the Indo-Gangetic plain affords perhaps the classic, although a not 

 altogether typical, example. The deposits of such regions are necessarily 

 hybrids between those of the desert on one side and the humid tropical 

 region on the other. The thorough chemical decay of rocks in well 

 drained hills subject to a pronounced rainy season yields ferruginous 

 clays, normally of bright red color. These are carried down and de- 

 posited on deltas and floodplains which are hot and parched for several 

 months during the year. On such plains a rank growth of vegetation is 

 not favored, and the sun burns out any residue of decayed organic matter 

 that may develop in the rainy season. During the long droughts, when 

 the water table sinks several feet, or even meters, below the surface, the 

 most recently deposited layer of silt becomes thoroughly dry and aerated. 

 The sediments, therefore, tend to remain oxidized, and even to become 

 dehydrated. Hence red and brown colors are the rule. Deposits of this 

 kind are characterized by abundant sun-cracks, being perhaps excelled in 

 this respect only by the playa deposits of the desert. Among fossils the 

 footprints of land vertebrates may be common in deposits of post-Paleo- 

 zoic age. Fish remains appear in extraordinary numbers, but only in 

 certain rare layers — probably an indication that overflow lakes from the 

 rivers of the wet season had subsequently shrunk until the fishes were 

 left to perish in the mud of the last remaining pools. The associates of 

 the river sediments are distinctive. Peat is scarce, but on the other hand 

 saline deposits are not typical of either monsoon or doldrum regions. 



