MEMOIRS 



OF 



THE GEOLOGICAL SURVEY OF INDIA. 



On a peculiar form of altered peridotite in 

 the Mysore State, by Thomas H. Holland, 

 A.R.C.S., F.G.S., Officiating Superintendent, Geological 

 Survey of India. 



I.— INTRODUCTION. 



Attention has been frequently called to the peculiar nature of the 

 secondary alteration exhibited by peridotites at various points in 

 South India. 1 



The commonest instances of these peridotite exposures consist 

 largely of dunites seamed with veins of magnesite, which vary -in 

 thickness from a few feet across to microscopic infillings of fissures, 

 cutting through the olivine-rock in all directions, and accompanied 

 sometimes by chalcedonic quartz and picrolite. As the result of 

 weather denudation, these veins of magnesite stand out as ribs, and 

 give the surface of the exposures a peculiar rough surface, made the 

 more noticeable by the general absence of vegetation which charac- 

 terises such areas, and for which of course such highly magnesian 

 rocks, devoid practically of the common elements of plant-food, give 

 rise to an unsuitable soil. Numerous, dazzling-white masses of 



1 Holland : " The comparative actions of subaerial and submarine agents 

 in rock decomposition." Rept. Brit. Assoc, 1898, p. 868; Geol. Mag., Jan. 1899, 

 p.*3o. " A contribution to the discussion on rock-weathering and serpentinization. " 

 Geol. M ag., December 1899^.540. " On the geology of the neighbourhood of 

 Salem. " Mem. Geol. Surv. Ind., Vol. XXX, pt. 2 



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