10 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



etc. This is largely due to the normally low water-table, which permits the 

 penetration of the air deeply into the soil, and the exposure of the mineral 

 constituents to oxidation or carbonation.^ Also, the lack of vegetation on 

 an arid flat means a lack of carbon, so that there remains a larger proportion 

 of ferric oxide, which requires the action of CO2 as an intermediate step 

 to its solution and removal, and a lack of reduction of the other higher 

 oxides, sulphates, and carbonates. The common result is the prevalence of 

 a red color,' the presence of gypsum associated with the remains of terrestrial 

 animals, and a lack of plant remains. Wind-ripples, tracks of animals, 

 sun-cracks, etc., are common but not always conclusive. The author well 

 remembers his surprise upon examining a wind-rippled surface of desert 

 sand in Arizona to find many markings which he would have unhesitatingly 

 pronounced worm-markings upon the wave-rippled surface of a marine 

 sandstone if the surface had been found fossil, but here the ripples were 

 formed by the wind — were forming as we watched — and the wormlike mark- 

 ings were formed by burrowing insects creeping just below the surface of the 

 heated sands, leaving trails similar to those formed by moles working their 

 way through a light soil ; nor was he able to detect a single criterion which 

 would have caused him to reverse his decision if the surface had been an 

 exposure of ancient sandstone. 



In flats formed in humid climates the higher water-table acts by prevent- 

 ing a free circulation of air and inducing a heavier growth of vegetation. 

 The excess of carbon and lack of oxygen permits the formation of the lower 

 oxides, hydrous oxides, sulphides, and even the native metals, such as copper. 

 The dominant colors are black, blue, green, yellow, from the presence of 

 compounds of iron low in oxygen. Traces of plants are frequent, and such 

 marks as footprints, rain-drop impressions, ripple-marks, etc., will be as 

 common as upon an arid flat. 



Alluvial fans and river deposits in general. — A detailed discussion of the 

 relation of alluvial fans, terraces, and river deposits to climatic conditions 

 in general has been given by Barrel and need not be repeated here, even 

 in part ; the student is referred to the paper as a type study of such con- 

 ditions.^ 



Delta deposits. — So full a discussion of the deposits has been given by 

 Barrell that only a reference to his articles is necessary.^ Dacque has 



1 A storehouse of information in regard to chemical changes will be found in Clarke, F. W., 



Data of Geochemistry, Bull. No. 616, U. S. Geological Survey, 1916; and Van Hise, 

 C. R., Metamorphism, Monograph 47, U. S. Geological Survey, 1904. 

 ^Tomlinson, C. W., The Origin of Red Beds, Jour. Geol., vol. 24, pp. 153 and 238, 1916. 



2 Barrel, Jos., Relations Between Climate and Terrestrial Deposits, Jour. Geol., vol. xvi, 



Nos. 2, 3, and 4, 1908. 

 * Barrell, Jos., Relative Geological Importance, etc., Jour. Geol., vol. xiv, Nos. 4, 5, and 6, 

 1906. Criteria for the Determination of Ancient Delta Deposits, Bull. Geol. Soc. 

 Amer., vol. 23, No. 3, 1912. The Upper Devonian Delta of the Appalachian Geosyncline, 

 Amer. Jour. Sci., vol. 36, November 1913, and vol. 37, January, March 1914. (This last 

 is a study of a type area.) 



