LAKE : GEOLOGY OF SOUTH MALABAR. 



granite is so intimate that in some specimens " it is not easy to pronounce which 

 is the enclosing substance," yet says that " the granite at the place where the 

 specimens were principally collected, appears to burst through an immense bed of 



the laterite, rising abruptly at a considerable angle," "in some specimens the 



two rocks are so mixed together as to form a sort of coarse breccia or rather conglo- 

 merate." 1 It is hardly necessary to remark that neither of the rocks is intru- 

 sive into the other and that the mixture is due to alluvial action. 



Hislop- in his Geology of Nagpur, contends that as laterite occurs on sand- 

 stone as well as gneiss, it cannot have been formed by the decomposition of the 

 gneiss. 



In 1859 appeared Mr. BlanfordV account of the Laterite of Orissa. He, 

 ike Lieutenant Aytoun, recognises two kinds of lateritic rocks, which he names 

 laterite proper and lithomarge. Wherever they occur together the former always 

 overlies the latter. Near hills it often contains pebbles, and it must therefore 

 be detrital. The lithomarge, on the other hand, has evidently been formed by 

 decomposition in situ of the gneiss. The quartzose layers of the gneiss extend 

 into it. On exposure the lithomarge becomes hard like laterite, Mr. Blanford 

 gives a series of analyses to show that there is a sudden decrease in the amount of 

 iron present in passing down from the laterite to the lithomarge, and that the 

 amount of iron in the lithomarge decreases with the depth till the water level is 

 reached. He concludes that the iron in the lithomarge is derived from the laterite 

 by the percolation of surface waters. 



He thinks that the laterite was probably derived from the magnetic oxide in the 

 gneiss. On decomposition the fine sand and clay are washed away and the 

 coarser sand and magnetic oxide remain. 



In the Trans. Bombay Geographical Society for i860, Dr. Buist 4 discusses 

 the laterite of Cochin and Quilon, which occurs with beds containing lignite 

 and is clearly sedimentary. He considers that this laterite has been formed by the 

 breaking up and redeposition of an older bed. He seems to agree with Aytoun 

 and others that laterite was originally formed by decomposition in situ of various 

 rocks. 



Messrs. King and Foote 5 in their Geology of Salem and the adjoining dis- 

 tricts recognise Mr. Blanford's two varieties of laterite, but the two occur quite 

 separately. The lithomarge is formed by decomposition in situ of the gneiss, for 

 the foliation is distinctly visible and the gradual transition from gneiss to litho- 

 marge is clearly seen. This rock is the same as the pseudolaterite of the Nilgiris 

 and Shevaroys. The laterite proper, on the other hand, is truly sedimentary. The 

 underlying gneiss is almost always perfectly fresh ; the gneissic rocks of the laterite 

 area are mostly quartzose or felspatho-quartzose and do not contain much iron ; 

 and the laterite contains rounded fragments (almost pebbles) of quartz. 



Up to this time laterite seems never to have been observed underlying any but 

 the most recent deposits. But Mr. Blanford in 1869 describes sections in the 



1 As. Res., XV, 178. 



2 Journ. Bo. As. Soc, V, 58. 



3 Mem.G.S. I., I, 280. 



4 Trans. Bo. Geog. Soc, XV, p. xxii. 



5 Mem. G. S. I., IV, 260. 



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