174 C. W. TOM LIN SON 



Bemmelen has shown that chemically prepared ferric hydrate, 

 after being partially dehydrated, if placed in a medium saturated 

 with water vapor, at ordinary temperature, takes up again part of 

 the lost water. 1 Brescius went so far as to say that "when nearly 

 dry, ferric hydrate has almost as great a tendency to take up water 

 as oil of vitriol itself." 2 This does not appear to be true, however, 

 of hematite found in nature. Once completely dehydrated, ferric 

 oxide becomes a stable compound. 



It remains to inquire into the means by which dehydration of 

 the more hydrous compounds might have been accomplished to 

 produce the low hydrates, in case these were not originally in the 

 same condition. The first agent of dehydration which presents 

 itself is that of heat. Elsden makes the following statement : 



The influence of temperature and moisture upon the iron hydrates is well 

 known. In the case of laterite in India, the yellow xanthosiderite soon weathers 

 to reddish-brown turgite, owing to dehydration. In the hot and arid regions 

 of South California the soils are dark red in colour, the iron being in the form 

 of hematite instead of the hydrous forms, gothite or limonite. Dehydration 

 also takes place in the hot regions of the Southern Appalachians, where the air 

 is comparatively humid. It is only the deeper portions of the soil which retain 

 the iron in a hydrated form. 3 



In all of the above mentioned cases, the source of the heat which 

 produces the reaction in question is the sun's rays. Its action in 

 the soils is limited to a superficial stratum rarely more than 15 feet 

 in maximum depth. 4 Obviously, the direct influence of insolation 

 cannot be responsible for any extensive dehydration in the Red 

 Beds, whose characteristic colors are known to extend to depths of 

 more than 2,000 feet. 5 



1 J. M. Van Bemmelen, "Sur le colloide de l'oxyde ferrique," Recueil des travaux 

 chimiques des Pays-Bos et de la Belgique, VII (1888), 112. 



2 E. Brescius, "Researches on Ferric hydrate," abstract in Jour. Chcm. Soc. 

 London, XXIV (1871), 497. 



3 J. V. Elsden, Principles of Chemical Geology (London: Whittaker & Co., 1910), 

 p. 97. 



4 W. O. Crosby, "On the Contrast in Color of the Soils of High and Low Lati- 

 tudes," Amer. Geologist, VIII (1891), 74; E. A. Smith, Geol. Survey of Alabama, Rept. 

 for the Years 1881 and 1882, p. 186. 



5 See p. 168. 



