Decomposition of Bodes. 31 



similarly decomposed at the surface, but, on account of the limited 

 amount of running water, the lime is retained in the decomposition 

 products, and forms a concretionary ' kankar.' 



In all these cases, however, although the action of the atmosphere 

 is so striking, the results are purely superficial, and a specimen of 

 rock taken from within a few inches of the clay products seldom 

 shows a trace of hydrous decomposition, even in thin sections under 

 the microscope. This is just as true for such delicate minerals as 

 olivine and nepheline as for the commoner silicates. In many of 

 the basic dykes, certainly pre-Cretaceous and probably Lower 

 Palaeozoic in age, the absence of serpentine is so complete that 

 unusual precautions are often necessary for the determination of 

 the olivine, whilst in the numerous occurrences of dunite throughout 

 the Madras Presidency serpentine is extremely scarce. In a nepheline 

 sj'enite recently discovered in the Coimbatore District, and at least 

 of Cuddapah age, the nepheline on microscopic examination shows 

 mere traces of alteration along the fracture cracks. 



In the light of European experience, where most of our petro- 

 graphical data have been established, the peculiarities of the Madras 

 rocks call for some special consideration, and the object of this paper 

 is to suggest that the probable explanation of the peculiarities now 

 referred to arises from a contrast of the geological histories of 

 the two areas. In Europe all, or nearly all, the rocks have been 

 submerged below the sea during the later geological periods ; in 

 South India there is no evidence beyond the immediate precincts of 

 the coastline of any depression below sea-level since Cuddapah 

 (probably Lower Palaeozoic) times. In Europe, therefore, the 

 features generally attributed to weathering are the compound 

 effect of submarine and subaerial action ; in South India the 

 former class of agencies has not affected the rocks now exposed, 

 and the remarkable freedom from hydration which the}' show 

 suggest that the action of the atmospheric agencies is purely 

 superficial. 



Taking into consideration the presence of lime carbonate and 

 other salts, with a larger proportion of carbonic acid and the great 

 pressure under which sea- water attacks a submerged rock-mass, it is 

 theoretically to be expected that submarine agencies are more potent 

 means of decomposition than those of the atmosphere ; but these 

 South Indian observations tend to show that serpentine and other 

 forms of hydrated products within rock-masses are due only in 

 a very limited degree to true -weathering. 



The products of atmospheric action are removed from the rock 

 surface as fast as they are formed, and deeper portions are fari passu 

 brought to the surface. It is not improbable that it is on account of 

 this denudation, which has proceeded Avithout known interruption 

 for so many geological ages, that relatively deep-seated portions of 

 the earth's crust have been brought to the surface in Madras, and 

 that the crystalline rocks there met with at times present peculiarities 

 for which European experience hardly prepares us. 



