NILGHIEI HILLS. 235 



are but little liable to decompose, and, on the other hand, those which 



contain no hornblende, although much felspar be present, are generally 



but little affected by the atmosphere. 



The most decomposable variety of the gneiss is that which has been 



Spheroidal weather- already described as occurring over the central 



°' part of the hills, and weathering into spheroids. 



This spheroidal decomposition does not appear to be caused by any 



peculiarity in the intimate structure of the rock. 



Due to jointing. 



but rather to be the consequence of the jointing 

 by which the rock has been broken up, probably during the upheaval 

 of the hills, as a secondary effect of the great forces which produced 

 the dislocations treated of in the preceding section. The weathering 

 of the rock commences at the sharp edges and angles of the jointed 

 fragments, gradually rounding them off, until the cuboidal mass, throw- 

 ing off coat after coat of decomposed rock, tends to assunie a spheroidal 

 form. The original joints are soon obliterated in the decomposed mass, 

 and the result is a number of spheroids of hard rock, of various sizes, 

 remaining imbedded in a sandy clay more or less ferruginous, and still 

 retaining traces of foliated structure, when such structure has been ori- 

 ginally very distinct* 



When a rock is very hornblendic, such as occurs over the tract mark- 

 „. ,, , ,, ,. ed on the map as " Hornblendic Gneiss," the iron 



Highly hornDlenaic ^ 



^°°'^s- originally contained iu the hornblende becomes 



thoroughly peroxidised and hydrated, and by a species of crystallization 

 forms a mammillated coating of impure Limonite, (Brown Htematite) 

 exactly resembling the characteristic surface of laterite. At one locality 

 Hear the road from Ootakamund to Makurty, and about two miles from 

 the Pykara river, a highly hornblendic band of rock has become thus 

 partially decomposed, and the fragments rolling down a steep hill side, 



* See Report on the Coal-field of Talcheer, Pages 41, 42, where two drav.ings are 

 given, illustrating this peculiarity of weathering. 



