482 A TREATISE ON METAMORPHISM. 



there is large pore space and abundant room, so that hydration may take 

 place without lifting the rocks, it is probable that with sufficient time the 

 process may go on to completion in all parts of the belt of weathering. How- 

 ever, in proportion as the rocks are dense and contain small pore space the 

 process of hydration at an early stage is likely to be retarded or altogether 

 stayed, because of the necessity demanded for more space and because the 

 chemical affinity of rocks for water is not strong enough, at least in the later 

 stages of the process, to overcome considerable pressure. 



While hydration is the rule for the heft of weathering, in regions in 

 which the temperature is high and which are not continuously humid dehy- 

 dration may take place. Dehydration probably occurs on the largest scale 

 in regions of high temperature in which dry and wet seasons alternate. 

 During the wet season hydration occurs, and at the dry times dehydration 

 occurs. This process is well illustrated by iron. As is well known, ferric 

 iron in the belt of weathering is ordinarily hydrous, and this gives a yellow 

 color. In regions of -high temperature where the humidity is low for at 

 least a portion of the year the soil is likely to be red, the iron being in the 

 form of hematite rather than gothite or limonite. Such regions are illus- 

 trated by the Desert Rang-es of southern California, in which dark red is the 

 dominant color. But dehydration also takes place to some extent in such 

 humid regions as the southern Appalachians. Crosby, Dana, and Russell 

 attribute to dehydration the bright-red color of the surface soil in this 

 region as compared with the less brilliant color of the subsoil. " This region, 

 although one of large precipitation, is apt to be dry in late summer and 

 autumn, the season when conditions are favorable to dehydration. In the 

 subsoil the process is naturally less marked. The dehydration of iron is 

 well known, because the process involves a change of color. Under the 

 conditions in which dehydration of iron takes place, it is certain that many 

 of the other hydrates which form in the belt of weathering are also dehy- 

 drated to a greater or less extent. It is well known that the process of 

 dehydration of many minerals begins at temperatures lower than 110° C. 

 Indeed, it is certain that the dehydration of limonite and other hydrous 

 oxides of iron begins as low as 100° C, and that if this temperature be 

 maintained even during the insignificant time in which an experiment is 



a Merrill, oit, p. 385. 



