164 G. P. MERRILL — WEATHERING OF MICACEOUS GNEISS. 



the upper zone, that above the water level and which may be called the 

 zone of oxidation, the weathering is manifested by a physical breaking 

 down of the rock mass as a geological body, whereby it is resolved into 

 gravel, sand, silt, and clay; a more or less complete change in the character 

 of the chemical compounds usually accompanies this disintegration. 



In this latter process there is invariably a tendency toward simplifica- 

 tion, complex silicates being broken up and their various elements re- 

 combined as simpler hydrous silicates, carbonates, oxides, sulphates, 

 free silica, etcetera. It is to a certain extent comparable with the entire 

 demolition of a building and the utilization of the old material in the 

 construction of new ones of quite different architectural types. The 

 deeper-seated process, on the other hand, is not destructive, but is to a 

 certain extent comparable with that of here and there substituting new 

 materials for old in a structure the general features of which are not 

 otherwise changed. New minerals are formed to replace, wholly or in 

 part, those that preexisted, but the rock retains its geological identity, 

 and in many cases is rendered even more enduring when actually ex- 

 posed to weathering influences than before. Among the minerals thus 

 formed are zeolites, epidote, amphiboles, feldspars, free quartz, chal- 

 cedony, etcetera. 



It is obvious that we have here two distinct processes which, while 

 they may grade into each other, are widely separable in their extremes, 

 both as to methods and results. Both unfortunately have been fre- 

 quently grouped under the general name of weathering and still more 

 frequently under the far more general name of decomposition. 



It would seem to the present writer that the name weathering should 

 be limited to those processes going on within the zone of oxidation and 

 resulting as a rule in the destruction of a rock mass as a geological body, 

 while the more deep-seated processes, which are really of mineralogical 

 rather than geological moment, should be looked upon as metamorphic. 

 Hydrometamorphism is sufficiently explicit, though the process in many 

 instances is metasomatic. Roth * makes some attempt at discrimination 

 in calling the deep-seated process " complicirte verwitterung," in distinc- 

 tion from the ordinary superficial "verwitterung; " but this name can 

 scarcely be considered as satisfactory, much less so in fact than our 

 term alteration, as commonly used. 



Formation of Zeolites in Soils. 



Second, as to the formation and existence of zeolites in the soil. From 

 all that is known regarding the actual formation of zeolites, it appears 

 that moisture and an absence of oxidizing agents is essential. They are 



* Allegemeine und Chemische Geologie, vol. i, p. 159. 



