W. J. McGee—The Laterite of India. 311 
into lithomarge, less ferruginous below, and finally into the clay 
formed by the decomposition of the subjacent rocks. Two varieties 
are recognized: high-level laterite, and low-level laterite ; but the 
one merges into the other. The low-level variety is supposed to 
have been washed down from above and recemented; but in some 
cases in which it is interstratified with Eocene beds (as at Surat) 
it cannot be more modern than the high-level variety. It occurs in 
perfection on the Deccan plateau, where it sometimes reaches a 
_ thickness of 200 feet. Most commonly, but not invariably, it rests 
upon igneous rocks. 
Laterite has, by different observers, been attributed to chemical 
change, in siti, of the volcanic rocks on which it usually reposes ; 
but this hypothesis does not fill all requirements. The hypothesis 
provisionally offered by the authors of the present “ Manual” was 
partially suggested by the late Sir Charles Lyell. The deposit is 
supposed to consist of metamorphosed volcanic ashes, scoriz, and 
tufas. The geological age of the high-level variety is hence 
(inferentially) supposed to be somewhat less than that of the Deccan 
traps (which were ejected about the close of the Cretaceous) ; but 
the low-level variety, which sometimes contains human relics, is 
believed to be later Tertiary or recent. 
Difficulties of the hypothesis—The above explanation is only offered 
as a “possible hypothesis,” and its difficulties are stated with that 
candour, deference to the opinions of others, and evident fairness, 
which characterize the work throughout. Some of these difficulties 
seem almost if not quite fatal. Thus the laterite overlies and seems 
to merge into igneous, metamorphic, and sedimentary rocks alike, as 
well as unconsolidated alluvial deposits; and in some of these cases 
the exposures are at high levels and far from eruptive centres, as 
near Gwalior, and in Bundelkhand. Furthermore, there are in 
Abyssinia large tracts of similar rocks to those of the Deccan, which 
are without any trace of laterite. Moreover, the constituents of the 
rock must have been subjected to very different orographical con- 
ditions at different localities (at Guzerat and Cutch, for instance, 
where it is of sedimentary origin, and interstratified with early 
Tertiary deposits, as compared with the summit of the Sahyadri 
range), which would be inimical to their similar metamorphosis if , 
they were so incoherent as volcanic ashes and scoriz. Again, the 
present disposition of the laterite does not seem to be that which a 
friable deposit, subject to long-continued erosion before consolidation 
took place, would assume. Once more, pisolitic iron ore, apparently 
homologous with laterite, is found both in Mesozoic sedimentary 
strata, as in the Rajmahal (Upper Gondwana) group (where it caps 
carbonaceous shale) and in Post-Tertiary alluvium, as at Calcutta, 
and occasionally throughout Bengal and Behar. Finally, the hypo- 
thesis does not explain the pipes, or tubular masses. 
Ferriferous Deposits of the Upper Mississippi Basin.—These de- 
posits, it is true, seem insignificant when compared with the Indian 
laterite; but it is at least probable that they differ from that mineral 
not in kind, but only in degree of development; and they are such 
