1855.] GILCHRIST RED SOIL OF INDIA. 553 



Red-Soil formation, and have made repeated search in it for organic 

 remains, but without success. 



The Origin of the Red Soil. — The Red Soil of Southern India is 

 derived from the disintegration of the ferruginous hornblendic rocks 

 so abundant and conspicuous in Peninsular India. The portions of 

 these rocks that have resisted the decomposing agents under the 

 influence of w^hich the more easily decomposable parts have given 

 way constitute ridge-like ranges of hills, extending in a direction 

 nearly N.W. and S.E., continuously or in detached hills ; but in the 

 latter case, the individual hills have a common compass-direction. 

 These hills, in consequence of their dark colour, contrast strongly 

 with the lighter-coloured adjoining surfaces, above which they do not 

 rise higher than 500 feet, seldom more than 200 or 300 feet. Their 

 declivities are steep and rough, consisting of blocks of rock, of from 

 one to six feet of cubic contents, and of parallelopipedal or spheroidal 

 shape. At the lower part of the declivities, the interstices between 

 these blocks are more or less filled up with reddish soil ; but the 

 ridges of the hills consist only of the blocks (the interstices being 

 unoccupied with soil), heaped together in a confused manner. 



With the exception of about half an inch or so inwardly from the 

 surface, these blocks consist of dark-blue, confusedly crystalline, 

 tough, hornblendic rock ; and this nearly disintegrated crust is of a 

 reddish colour, but of a more yellowish hue than the colour of the 

 red soil. 



These hills are unmistakeably of a volcanic origin, and have broken 

 up the previously existing granitic gneiss, with which, in colour, they 

 strikingly contrast, and intersecting the undulatory or hilly surface 

 of which they are frequently seen stretching in straight lines as far 

 as the eye can reach. 



In making sections on the lower parts of the declivities of these 

 hills, or cutting into surfaces nearly level with the adjoining ground, 

 in order to make a level line of road, some of the following varieties 

 of lithological structure in these trap-rocks have presented them- 

 selves : — 



(a) A jointed structure ; the fragments having a more or less 

 rhomboidal shape (concretionary rhomboids), and frequently in a 

 state of partial disintegration, sometimes quite decomposed ; colour 

 red, though not so dark as that of the red soil. Occasionally this 

 structure occurs with the centres of the individual rhomboidal con- 

 cretions consisting of fresh undecomposed rock, surrounded with de- 

 composed cases more or less thick. 



(b) Spheroidal masses of concentric structure, more or less decom- 

 posed at surface, but with centres of fresh rock, extremely tough, 

 and in size varying from one inch to one or two feet or more in 

 diameter. The interstices between these spheroidal concretions or 

 nuclei vary considerably, from one inch to two feet or more ; the 

 interstices occupied with red earth or soil*. 



* I find at Rothesay (1854) a trap -rock, on the western side of the dam of 

 Lochfad, exhibiting exactly the appearance of the variety b, above described. On 



