AGENTS OF WEATHERING. 481 



HYDRATION AMI DEHTDEATION. 



By hydration is meant the union of water with chemical compounds, 

 thus producing hydrous minerals. The water for hydration is derived 

 mainly from the hydrosphere. Hydration stands as the most extensive 

 reaction in the belt of weathering-. In its importance in this belt as a 

 geological process it is second only to carbonation. Indeed, it has been 

 supposed by some geologists that hydration is the dominant reaction of the 

 belt of weathering, the process of carbonation being wholly ignored. The 

 list of important hydrous minerals formed comprises many silicates and 

 oxides and some carbonates, sulphates, etc. 



As a matter of observation, all of the so-called anhydrous silicate 

 minerals of the igneous, sedimentary, and metamorphic rocks which have 

 long remained in the belt of weathering- are shown by analysis to have 

 become more or less hydrated. But more important than this, the new 

 minerals which develop in the belt of weathering are usually strongly 

 hydrated. Of these in the silicate class the kaolin, serpentine-talc, chlorite, 

 and zeolite groups are examples. Of the oxides the most important are 

 those of aluminum and iron, the former occurring as gibbsite or diaspore; 

 and the latter commonly as limonite, but not infrequently as gothite or 

 some other hydrated oxide. 



In many cases in the dense rocks hydration goes on to a certain stage 

 and then ceases or at least becomes very slow. This is due to the fact that 

 the process of hydration involves expansion of volume and therefore makes 

 it necessary that the superjacent material be elevated if the process continues. 

 Such partly hydrated rocks below the surface when brought to the surface, 

 and therefore relieved from pressure, may continue to rapidly hydrate nearly 

 or quite to the completion of the process with great expansion of volume. 

 In many cases the process is so rapid that the term slaking is applicable. 

 This slaking has been observed by Merrill in the granitic rocks of the 

 District of Columbia, ° and by Derb3 r in the sedimentary rocks from rail- 

 way cuttings of Brazil. h In both of these places the rocks when exposed 

 at the surface soon break into powder, although in position they are perhaps 

 so strong as to require blasting. These facts make it clear that the process 

 of hydration is largely dependent upon the pore space in the rocks. Where 



° Merrill, George P., Rocks, rock-weathering, and soils, Macmillan Co., New York, 1897, p. 188. 

 ''Derby, O. A., Decomposition of rocks in Bi-azil: Jour. GeoL, vol. 4, 1896, pp. 529-540. 



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