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MANGANESE DEPOSITS OF INDIA: GEOLOGY. [ PaRT IT: 
of detrital origin, having been formed by the denudation and reconsoli- 
dation of the typical rock ; but there is no need to consider this detrital 
variety of high-level laterite here, except to say that it is often litholo- 
gically indistinguishable from the true low-level laterites. 
The origin of the high-level laterites is a much debated question, con- 
ceming which many hypotheses have been advanced. This divergence 
of opinion is partly due to the fact that this rock is eminently 
susceptible of segregative changes, which produce a constant rearrange- 
ment of its constituents with the obliteration of all original structures ; 
and partly due to what seems to be a fact, namely that two distinct 
South Indiau ^YV^^ ^^^^ have been designated by the term 
variety of high-level ' high-level laterite '. With regard to one of these 
MedUcott and Blanford remark l;— 
' ferruginous clays, with but little of the true character of I iterite, and due 
solely to the decompcsition of gneissic rocks, have bjen occasiomlly described 
under the name, uch is certainly the case with the Xilgiris, one of the Iccalitips 
mentioned by several geologists. Xo well-authenticated c.ccurrence of laterite is 
known at an elevation exceeding 5,000 feet above the sea '. 
Dr. HoUand ^, however, whilst aware of this passage, prefers to 
regard this rock as laterite. Such laterite, as might have been anticipated 
from its mode of derivation, often contains a considerable amotmt of silica, 
both free and combined. It seems to prevail in the areas occupied by the 
gneissose and crystalline rocks in the southern parts of the Peninsula, 
and often preserves the original structures of the underlying rock, pisolitic 
or concretionary structures being rare 3. 
The second type of high-level laterite is that which occurs in the 
The Deccan variety form of horizontal caps, resting usually on rocks 
of high-level laterite. of the Deccan Trap formation, but not infrequently 
on older rocks, especially the Dharwars. Those patches not actually on the 
Deccan Trap are usually situated near, but sometimes at great distances 
from, the present edge of this formation. This type of laterite does not 
show any structures that can be compared with corresponding structures 
in the underlying rock, and is not infrequently markedly pisolitic. The 
impression conveyed by the appearance of the often vertical scarps of 
this rock, particularly when the underlying rock is trap, is that there is 
not a gradual downward passage from completely-formed laterite into 
1 ' Manual of the Geology of India ' (1879), p. 35G. 
2 Oeol. Mag., Dec. IV, Vol. X, p. 63, (1903). 
3 I did not myself happen to notice any laterite in the Nilgiris that preserved the 
original structure of the gneissic rocks. The materials that did show this were the 
ferruginous clays (and lithomarges) referred to by Medlicott and Blanford. 
