542 STUDIES FOR STUDENTS 



Besides these general stratigraphical relations which should 

 characterize the mud-cracked deposits of arid flood-plains may be 

 mentioned other associated characteristics, some of which are 

 pointed out by Walther/ Such deposits are usually rather barren 

 of fossils of water- living forms; the latter, if present, are apt to be 

 restricted to the lines of sandstone which mark the ancient channels^ 

 or to the deposits of shallow lagoons. The flood plain proper is 

 more likely to contain the remains of air-breathing forms, but as 

 conditions must have been frequently unfavorable for their life or 

 for their preserval after death the strata are more usually barren. 



Further, land deposits on account of the local and annual varia- 

 tions of conditions are apt to show various sorts of deposits — water 

 borne, wind borne, organic, and volcanic, in close association but 

 differentiated from each other. Marine deposits are not subject 

 to this rapid variation and more gradual transitions are observed. 



Deposits formed in rivers or in lakes and seas have usually green- 

 ish or bluish shades of color as in marine deposits. Those sub- 

 jected to subaerial exposure, however, under arid or subarid condi- 

 tions are apt to possess a normal content of iron owing to the absence 

 of carbon and the opportunity for complete oxidation following 

 the subsidence of the ground water. The river muds from which 

 the iron has not been leached by the deoxidizing influence of vege- 

 tation may thus be yellow, brown, or red. In well-lithified but still 

 unmetamorphosed formations, in which the iron still exists in the 

 form of a free oxide, reds predominate, whereas in modern muds 

 derived from the erosion of granite lands yellow or brown is observed 

 to be the prevailing color. But Crosby^ has shown that a gradual 

 dehydration of the ferric oxide serves to transform colors originally 

 yellow and brown into deep red or vermilion. 



River deltas normally contain abandoned channels or lower tracts 

 of country not yet built up which are more or less permanently flooded 

 with fresh water. Such are usually the seats of luxuriant vegetable 

 growth and abundant animal life, even under climates where the 



1 Einleitung in die Geologie (1893), pp. 719-26. 



2 J. B. Hatcher, "Origin of the Oligocene and Miocene Deposits of the Great 

 Plains," American Philosophical Society, Vol. XLI (1902). 



3 "On the Contrast in Color of the Soils of High and Low Latitudes," American 

 Geologist, Vol. VIH (1891), pp. 72-82. 



