PRODUCTS OF ROCK DECAY 327 



Its appearance and doubtless also its composition vary somewhat from 

 place to place. The bright red is varied by shades of yellow, brown, 

 and occasionally olive green, but the prevailing tint is nevertheless very 

 generally red. The abrupt change in color between the residual clay 

 and the adjacent alluvial clay is very striking. The latter is never red, 

 but is always some shade of gray or blue. The only essential difference 

 between the two clays is in the form of their iron. In the alluvium this 

 is in the ferrous state, forming light colored compounds. In the residual 

 clay it is in the ferric state, and not only more highly oxidized, but the 

 oxide is in large measure dehydrated, giving the bright red color of 

 hematite. 



The cause of this difference in the state of oxidation in clays which 

 appear to be affected b}^ the same conditions is doubtless the different 

 amounts of organic matter incorporated with them. 



As already described, the residual clay is very compact. It is never 

 loosened by frost or by shrinkage cracks. The only means by which 

 vegetable matter finds its way below the surface is by growing roots 

 and insect burrows. The amount thus introduced is not sufficient to 

 materially affect the chemical conditions within the zone of rock de- 

 cay. The vegetable matter at the surface is so rapidly and thoroughly 

 oxidized that the organic compounds which result from the process are 

 not effective reducing agents when they percolate downward in contact 

 with the red clay, but probably carry an excess of oxygen which is ex- 

 pended in the oxidation of the rock constituents below. In the alluvium, 

 on the other hand, the vegetable matter while only rarely constituting 

 a large proportion of the mass, is thoroughly disseminated through it, 

 and controls the chemical conditions preventing the oxidation of ferrous 

 compounds and reducing ferric compounds to the lower state of oxida- 

 tion. Before the deposition of the alluvium which now fills the valleys 

 of the region the bottoms of these valleys were covered with residual 

 cla}' the same as that now covering the hills. This clay underlying the 

 alluvium and subjected to the constant downward percolation of the re- 

 ducing solutions from the latter has generall}^ though not always lost its 

 red color. It is often found to be mottled with blue patches where the 

 reducing solution has gained access to the ferric oxide. 



Doubtless the proportion of silica, alumina, and iron depend to some 

 extent on the composition of the rock from which the clay was derived, 

 but this variation is not sufficient to produce marked differences in its 

 appearance and physical properties. The depth of this upper division 

 is not very great, usually from 10 to 30 feet. The separation between the 

 red clay and the underlying blue clay is usually rather sharp, although 

 in many cases there is a band of mottled clay between the two. 



