312 W. J. McGee—The Laterite of India. 
as to throw some light on their mode of origin. The deposits are as 
follows:—1. In alluvium and upper till. In fertile sloughs and 
rich alluvial bottoms the boulders and pebbles are often incrusted, 
and thin layers of sand and gravel are sometimes cemented, with 
ferric oxides. There are also in like situations, pisolitic nodules 
(compact throughout) of peroxide of iron irregularly disseminated, 
pipes of the same evidently formed around roots of grasses, ete., 
occasional thin bands of bog-ore, and ferruginous concretions (a 
crust of limonite without—ochreous and incoherent within), which 
may be found in any stage of formation. Rarely, as below rich 
pastures or barnyards, the upper till is impregnated with protoxide 
of iron. 2. Lower till. The whole formation is rich in iron, too 
generally disseminated to be of economical importance. It is 
coloured by the protoxide. It occasionally contains deposits of 
impure bog-ore, some feet in thickness, small masses of pyrites, etc., 
and cylindrical ferruginous concretions (distinct from both the 
above—evidently formed around roots), hollow or containing a 
woody core, and exhibiting the concentric structure of exogenous 
wood. The same horizon is highly carbonaceous, also, containing 
much wood, peat, lignite, ozokerite, etc. 8. Subjacent formations. 
Below the lower till a ferruginous deposit of local and erratic 
materials sometimes occurs. It may be either a lithomarge or a 
conglomerate of sand and northern pebbles in a matrix of earthy 
limonite. The products of the secular disintegration of the sedi- 
mentary strata are similarly ferruginous—much more so than the 
unaltered rock—and contain limonitic concretions. Within the 
cavities of the Paleeozoic rocks considerable masses of iron are some- 
times found, which resemble the smaller concretions of the upper 
till in structure and material, and probably also in origin and— 
approximately —in age.'. These are not to be confounded with inter- 
stratified ore-bands. 
Briefly, the Mississippi valley ores are attributed to the agency of 
decomposing vegetable matter in liberating the iron from adjacent 
rocks and earths, and the subsequent combination of this iron with 
the atmospheric oxygen. This property of vegetal solution was 
long ago pointed out by Dr. Sterry Hunt. Accordingly, the iron 
is most abundant in those formations and localities in which the 
decomposition of vegetable matter has been greatest. 
Application of the hypothesis to the lateritic formations.—In accord- 
ance with the principle indicated above it may be suggested that the 
Indian laterite is the product of alteration in siti of the underlying 
rocks by the usually recognized atmospheric and chemical agencies, 
modified by the action of decomposing vegetation. In consequence 
of the operation of this last factor, all the iron of the rocks has 
probably been retained near its original position (and possibly 
increased through metasomatism), while other elements have been 
borne off in the winds or carried down to the sea. The accumula- 
‘ For more complete description, see papers by the author in Proc. Am. Assoc. Ady. 
Sci., 1878, and in Gor. Mac. for August and September, 1879, as well as the 
different State Reports. 
