44 LAKE : GEOLOGY OF SOUTH MALABAR. 



ary origin. Mr. Foote infers from the conglomeratic nature of this Chapoli iron 

 clay that it is either wholly or in part an altered intertrappean pebbly clay. 



The iron clay occasionally overlaps the trap area and rests directly on other 

 rocks. In the Gauli plateau it is found lying on gneiss. There is no external dis- 

 tinction between the iron clay found overlying trap and that found on gneiss. The 

 passage downwards of the base of the clay into the gneiss is clearly seen. 



The chief difficulty in Mr. Foote's view is, as he points out, that the agates and 

 geodes so common in the trap-flows are never found in the summit bed of laterite. 

 Mr. Fcote supposes that the uppermost trap-flow, from which the summit bed was 

 formed, may have been free from geodes. Such flows are not very uncommon in 

 the Deccan traps, though the majority contain geodes. 



The laterite of the Konkan 1 is believed by Mr. Foote to be sedimentary. It 

 lies unconformably on the trap, and at Ratnagiri it overlies certain clays with lignite, 

 resin, and plant remains. These clays are of Eocene age and clearly of sediment- 

 ary origin. 



The laterite of the Rajmehal Hills is briefly described by Mr Ball. 2 There 

 are at least two varieties, one occurring in patches on the metamorphic rocks west 

 of the hills, the other commencing with the western scarp of the trappean rocks and 

 stretching eastwards. This laterite gradually slopes downwards to the east and is 

 continuous with the laterite of the plains. Many sections show an intimate con- 

 nection between the laterite and the basalt, which suggests that the laterite may 

 sometimes be merely altered basalt. But there is no proof of this. 



In the Manual of the Geology of India, 3 (1879), Mr. Blanford givesja condensed 

 account of the laterite of India, which he divides into two groups — high level 

 laterite ( = iron clay of Voysey and Foote), and low level laterite. He discusses 

 the mode of origin of the rock, and concludes that both forms are sedimentary. He 

 suggests that the laterite may be the re-arranged and altered ashes, scoriae, &c, 

 which he considers were probably thrown out in great quantity at the close of the 

 Deccan Trap period. The laterites of the Guzerat nummulitics were, in all prob- 

 ability, formed of volcanic detritus washed down into the sea, so that this mode of 

 formation must at least be looked upon as possible. 



Mr. Foote, 4 in his Memoir on the East Coast from 15 N. L. to Masulipatam, 

 describes the lateritic rocks of that area as consisting of sands, gravels, and con- 

 glomerates — clearly sedimentary. 



In the Geological Magazine for July 1880 is a paper by Mr. W. J. McGee 5 

 which compares the laterite of India with the ferriferous deposits of the Upper 

 Mississippi Basin, and suggests that both may have been formed in the same 

 way. The Mississippi deposits are insignificant compared with the Indian laterite, 

 but the difference may be not in kind but in degree. " The Mississippi Valley ores 

 are attributed to the agency of decomposing vegetable matter in liberating the iron 

 from adjacent rocks and earth, and the subsequent combination of this iron with 

 atmospheric oxygen." Mr. McGee suggests that the Indian laterite also may have 

 been formed by the alteration in situ of the underlying rocks by atmospheric and 



1 Loc. cit., p. 224. 



2 Mem. G. S. I., XIII, 222. 



3 Manual of the Geology of India, I, 334 ; II, 563. 



4 Mem., G. S. I., XVI, 85. 



* Geol. Mag., New Series, Dec. II, Vol. VII (1880), 310. 



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