MATERIALS OF BELT OF WEATHERING. 521 



Missouri. Here the country rock is quartz -porphyry, in which are veins of 

 iron oxide. The quartz-porphyry was disintegrated, decomposed, and 

 largely removed, while the iron oxide was concentrated on the surface of 

 the mound, producing a residual mass composed largely of nodules and 

 bowlders of ferric oxide in a matrix of residual clay. 



Where the conditions for solution aud removal of the soluble material 

 are not favorable, in addition to the end products above given there may 

 be found important amounts of many other relatively soluble minerals. 

 This condition of affairs, as explained on pages 497-498, frequently occurs 

 in arid and semiarid regions, where in the residual soils are found consid- 

 erable quantities of carbonates, sulphates, chlorides, and nitrates of the 

 alkalies and alkaline earths. 



These ultimate products of weathering are found to the exclusion of 

 other compounds only where the process is complete, and this is very 

 exceptional. Even under very favorable conditions of weathering' the limit 

 of disintegration is never attained and decomposition is usually far from 

 complete. The end products of weathering are found as preponderant 

 constituents only in the soils and subsoils in humid warm regions, which 

 are permeated by plants; and even in these soils it is rare indeed to find all 

 the constituents fully changed to the ultimate products above given. In 

 proportion as the process of weathering is incomplete, other minerals and 

 compounds are present; and it has already been seen that all kinds of rocks 

 may exist in all stages of disintegration and decomposition. Hence the 

 chemical composition of the rocks of the belt of weathering may vary 

 most widely. Ordinarily the undecomposed or partly decomposed materials 

 are prominent or even predominant. The undecomposed minerals are 

 likely to be present in the order of their refractoriness. Therefore, the end 

 compounds above given adapted to the belt of weathering' may vary from 

 a very subordinate quantity to an amount which vastly preponderates over 

 all other constituents. While this is true, it is perfectly clear that the sum 

 total of the processes of weathering tends toward mineralogical and chem- 

 ical simplicity. The end products of weathering comprise but a few of 

 the mineral species, and these species are simple compounds of a few 

 elements." Hence the propriety of the assignment of this belt to a zone 

 of katamorphism. 



<OIerri!l, George P., Rocks, rock-weathering, and soils]" Macmillan Co., New York, N. Y., 1897, 

 pp. 265-266. 



