30 The American Geologist. juiy, isns 
In India there is a remarkal)le deposit of chiy. called laterite, 
the origin of which the Indian geologists have not yet definitely 
determined.* It is a red. sometimes mottled, porous, argillaceous 
rock much impregnated with iron })eroxide and sometimes resemb- 
ling jasper, but not so hard as a purely siliceous mineral. It is 
often traversed by tubes, sometimes vertical, sometimes horizon- 
tal, antl sometimes irregular, which are filled with clay. The rock 
when first (luarried is so soft that it can be cut out with a pick; 
but it hardens greatly on exposure. There are two kinds of lat- 
erite. the -'high level" and "low level," which in places seem to 
grade into one another, but which in their extremes are quite dis- 
tinct, for the former is rarely stratified, while the latter is most 
commonly banded and liears evidence of marine origin. The low 
level laterite will lie considered under the discussion of sediments 
derived from secular decay. 
The upper laterite often passes insensibly into the underlying 
rock, whether it is igneous, metamorphic or sedimentary, the 
transition form being called lithomarge. Sometimes, however, 
the dividing line between lithomarge and lied rock is quite marked. 
The laterite is quite indestructible and the region occupied by it 
is generall}' barren. Its thickness varies from fifty to two hundred 
feet. The high level laterite occurs as high as 4,700 feet above 
sea level and is found chiefly on the traps of the Deccan plateau, 
though it also occurs on the older rocks several hundred miles from 
any traps. It must formerly have been much more extensive and 
perhaps formed a continuous sheet, though now very much dis- 
sected by erosion. 
As to its origin, no definite conclusion is arrived at by the aitth- 
ors. though the low level laterite is believed to have been derived 
from the upper laterite partly by the action of the wind, partly b}' 
stream action. One suggestion in regard to the origin of the high 
level laterite is that it has been deriA^ed from the decay of a ba- 
salt sheet formerly more extensive. Numerous obections are 
found to this theory, chiefly the difficulty of accounting for such 
a sheet in many places where laterite is found. Another sugges- 
tion is that it represents the decayed product of a vast sheet of 
volcanic ash deposited during the formation of the Deccan traps. 
This theory the authors seem to think the most probable one thus 
*Medlicott and Blanford, A Manual of the Geoloijy of India, 1879, 
Vol. I, p. 348. 
