GENERAL CONSIDERATIONS. 201 



heat are not general, but are confined to the bands themselves. These 

 facts narrow the issue to the very natural inference that the heat 

 was produced by the process of brecciation itself, which was probably 

 of a much more violent nature than that which produces the com- 

 moner and more general phenomena of dymano-metamorphism. 



When such deformities as eye-structure, mortar-structure and 

 peripheral granulation of the constituents result from dynamo- 

 metamorphism, the whole, or a large portion, of the rock mass suffers, 

 and the disturbance being more general, the temperature rises only 

 to a limited degree. There are also good reasons for concluding that 

 a general deformation of all the constituents of a rock indicates a 

 gradual and slow application of the dynamo-metamorphic agencies. 

 In these cases, therefore, where the rock has smashed along a 

 particular band or line, the local rise of temperature resulting from the 

 heat of friction is likely to be excessive ; first, because of the limited 

 area to which the disturbance is confined, and secondly, because this 

 gives a prima facie reason for supposing that the disturbance must 

 have been unusually violent. 



Briefly, it is concluded that this peculiar phenomenon is the 

 direct result of brecciation, and is not due to actual injection of trap ; 

 the black colour and indurated nature of the mylonite (which is 

 outwardly so tachylytic in appearance) are probably the result of 

 the heat produced during the violent brecciation of the rock by 

 which a temperature sufficient to frit the mylonite, but insufficient 

 to melt the rock, was produced. 



The geographical distribution of this peculiar breccia and the 

 direction of the bands will probably prove to be of some geological 

 interest ; but so far all the cases which have been recorded 

 occur within the district of Salem. Messrs. King and Foote called at- 

 tention to the fact thai the bands often resist the weather more effec- 

 tually than the surrounding unaltered rocks, and so stand up as well- 

 marked ridges. Besides the case they Tefer to near Ahtur, a very 

 striking instance is exhibited at about half a mile north-east of Kag- 

 ankarai in the Tirupatur Taluk, running, like the Ahtur ridge, 

 G 2 ( 83 ) 



