176 C. I!'. TOMLINSON 



refers 1 appears to have considered the observed dehydration as 

 truly "spontaneous"; and certainly they have shown the process 

 to be closely dependent upon external conditions. The chemically 

 precipitated colloidal ferric hydrates possess, when first formed, a 

 higher content of water than any of the forms known to occur as 

 minerals. According to Van Bemmelen,- on standing in a dry 

 medium at ordinary temperatures these colloids gradually approach 

 the composition 2Fe 2 3 • H 2 0, beyond which the percentage of 

 combined water is not reduced without the application of much 

 higher temperatures. Under water, or in air of moderate humidity, 

 they have not been shown to lose water beyond the composition 

 Fe 2 3 - 2H,0 except at temperatures above 50 C., 3 and heating at 

 50-60 C. for 2.000 hours failed to bring about dehydration beyond 

 the composition 2Fe a 3 • H 2 0. 4 Temperatures as high as these 

 cannot be assumed to have existed, except locally, in the Red Beds 

 sediments since burial. The so-called spontaneous dehydration 

 observed in the laboratory is probably subject to the terms of Van 

 Bemmelen's conclusion: "The red-brown substance, which has 

 been considered to be a hydrate, is a colloid .... which has no 

 stable composition ; it maintains an equilibrium with the tension of 

 the water vapour in the surrounding medium." 5 Since burial, the 

 great mass of the Red Beds sediments, except in the most arid dis- 

 tricts of the West, have been saturated with ground-water, a con- 

 dition decidedly unfavorable to dehydration at the temperatures 

 there existing. 



Yet another agent of dehydration is mentioned by Elsden : 

 The presence of any substance in solution which lowers the vapour tension 

 of water will lower the inversion temperature of gypsum Even solid 



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

 chimiques dcs Pays-Bos ei de la Belgique, VII (1888), 106-14; Edward Davies, " Action 

 of Heat on Ferric Hydrate in Presence of Water," Jour. Chetn. Soc. London, XIX 

 (1866), 69-72; G. C. Wittstein, " Uber das Verhalten des Eisenoxyhydrates unter 

 Wasser," Vierteljahreschrift fur praktische Pharmacie, I. Band (1852), 275-76. 



2 Op. cit., pp. 110-11. 



3 T. Carnelly and James Walker, "The Dehydration of Metallic Hydroxides by 

 Heat," Jour. Chetn. Soc. London, LIII (1888), 89; D. Tommasi. "Ferric Hydrates," 

 abstract in Jour. Chetn. Soc. London, XLIY (1883), 24. 



1 Davies, op. cit., p. 70. 



5 Van Bemmelen, op. cit., p. 1 14. Translated from the French. 



