8G8 REPORT— 1898. 



5, The Comparative Actions of Suha'crial and Subniarine AgpMts in Rock 

 Decomposition. By Thomas H. Holland, A.R.C.S., F.G.S., Geo- 

 logical Survey of India. 



In Europe, nearly all crystalline and igneous rocks of any considerablo age 

 show signs of hydrous decomposition, which by the microscope can generally be 

 traced far beyond the limits of the very evident superficial crust of weathered "pro- 

 ducts : in some cases, like the peridotites, the changes due to hydration, even in 

 rocks of Tertiary age, have resulted in a practically complete alteration of the 

 original constituents. In working over various parts of Peninsular India, the writer 

 has been struck by an almost constant absence of any hut the most superficial traces 

 of hydration, even in minerals like olivine and nepheline, which are so noticeably 

 susceptible to the action of water. As in all tropical and moist climates, however, 

 a complete and rapid superficial decomposition is shown by most of the rocks, and 

 in some areas they are found to be changed into a ferruginous clay, which, though 

 forty or fifty feet thick, is found to retain the characteristic macroscopic structures 

 of the original rocks. In some districts, where the atmosphere is always warm, 

 and during the monsoon season highly charged with moisture without great preci- 

 pitation of rain, the rocks are 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-Oretaceous and probably Lower Palaeozoic in age, the absence of ser- 

 pentine is so complete that unusual precautions are often necessary for the deter- 

 mination of the olivine, whilst in the numerous occurrences of dunite throughout 

 the Madras Presidency serpentine is extremely scarce. In a nepheline syenite 

 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 ourpetrographical data have 

 been established, the peculiarities of the Madras rocks call for some special con- 

 sideration, 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 Em-ope aU, 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 coast-line 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 sub- 

 marine and subaerial action ; in South India the former class of agencies have not 

 aff'ected the rocks now exposed, and the remarkable freedom from hydration which 

 they show suggests 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 to 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 &re pari passu brought to the surface. It is 

 not improbable that it is on account of this denudation, which has proceeded with- 

 out 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 

 tlie crystalline rocks there met with at times present peculiarities for which European 

 experience hardly prepares us. 



