416 Geological Sketch of the Neilgherries. [Aug. 



vegetable earth, we see thick beds of a yellow ocbraceous earth 

 abounding with silica (No. 4). Indeed, in someplaces, as atKotagherry, 

 it resembles very much the yellow Venetian Tripoli, previous to under- 

 going preparation for the arts (No. 5). The geological position, how- 

 ever, of the two, differs very much — the Venetian Tripoli, which is 

 brought there from Corfu, and from the neighbouring coasts of Epirus, 

 is found (as I have had opportunities of ascertaining) in the sandstone 

 formation, which alternates with the magnesian limestone*. The 

 kind of Tripoli I met with on the Neilgherries, seems to be the result 

 of the disintegration of a species of iron flint found in primitive 

 formations; some of the specimens I collected, have a great resem- 

 blance to the Eisnkeissel of Werner (No. 5^). Some varieties of 

 the finest white Tripoli arise from the decomposition of silicious 

 rocks, such as calcedony, in Corfu and in upper Italy ; but in gene- 

 ral, the Neilgherry specimen is not so silicious, and seems to contain 

 a good deal of alumina and iron. It is in this yellow clay that 

 we occasionally see some tubular bodies, formed by concentric 

 layers of the same clay, round the numerous roots of plants that 

 grow on the soil above (No. 6). But what attracted my attention 

 most was, to see (at Kotagherry) those tubular bodies traversing the 

 thick stratum of black earth, which overlies the yellow clay, without 

 having a particle of it in their composition. As if the roots, by a 

 kind of capillary attraction, sucked up through the black soil, without 

 mixing with it, the particles of the yellow clay which, undisturbed by 

 the vicinity of the black soil, arranged themselves concentrically to the 

 root ; and the latter decaying has left the cavity of the tube emptyf. 



* It seems to be an argillaceous iron ore, similar probably to the one at Ash- 

 bumham, used for the manufacture of Tripoli, and belonging to the Hastings 

 sands. — See Fitton's Geological Sketch of the vicinity of Hastings, page 50. 



f " Brogniart alludes to something similar to these tubular bodies, enclosing 

 the roots of plants in sandy places, where the iron appears to aggregate the sand 

 round the roots ; and he concludes the paragraph by confessing his inability to 

 assign the cause producing it ' et dans ces-(;i la cause qui a accumule Poxide de 



fer a Ten tour de la racine est encore difficile a assigner. — Tail, des Terr, qui 



composent la Surface du Globe, page 56.' 



My friend Mr. Malcolmson, Secretary of the Madras Medical Board, writes 

 to me as follows : On the banks of many of the streams in the Deccan, the black 

 soil is seen penetrated by tubular incrustations, resembling kankar ,■ they are evi- 

 dently formed round tbe roots of plants, the decay of which leaves a cavity which 

 may sometimes be seen to divide and ramify. Some of those in the banks of the 

 Kanar river, Kamptee, near Nagpore, are more than an inch in diameter. — B. 



Sergeant Dean's Jumna collection exhibits many incrustations of calcareous 

 and ochreous matter of a similar nature. — Ed. 



