DESCRIPTION OF THE PRINCIPAL EXPOSURES, 185 



are of daily occurrence, whilst the vegetation is only allowed an 

 insecure and temporary place for development in places where, by 

 local accident, the slope is comparatively gentle. In the Nilgiris the 

 slopes are clothed with thick turf, and the valley " bottoms" often 

 covered with deep peat bogs. 



In consequence of this protection from rapid erosive action 

 the weathering: of the rocks has gone on by 



Weathering. , .... , 



simple decomposition in situ to depths often of 

 40 or 50 feet. Road cuttings through this decomposed material show 

 the average products to be a soft, yellow or red clay, in which, on 

 account of the variations in original composition, the basic portions 

 can, though reduced to a mere clay, be easily distinguished from the 

 acid contemporaneous veins which contain more pure kaolin and are 

 seen to ramify in all directions through the rock. Often in a neatly 

 cut slope of this clay small local faultings are beautifully displayed 

 with diagrammatic clearness. 



In the midst of this deep covering of decomposition products, 

 rounded boulders of the fresh rock are frequently found without any 

 apparent regularity of relation to their depth from the surface. The 

 onion-like shells of partially decomposed material around these boul- 

 ders are seldom more than a few inches thick, so the passage from 

 the pure clay to the fresh rock is remarkably rapid. It is to this in- 

 teresting feature that I have referred as an illustration of the differ- 

 ences between the decomposition produced in rocks exposed to the 

 action of unaided atmospheric agents, and the alteration noticeable in 

 rocks which have been depressed below the sea level and submitted to 

 the corrosive action of water charged with carbonic acid and salts 

 acting under pressure {vide p. 197). Sections taken from the rock of 

 the Nilgiris within a few inches of the clay decomposition products 

 are seen, on microscopic examination, to be most remarkably fresh, the 

 felspars being perfectly clear and free from kaolinization. The same 

 thing is true of the olivine-crystals in the norite dykes which cut 

 through the charnockite series in the same area. Even in small 

 boulders olivine-crystals are generally without the slightest signs of 

 F 2 ( 67 ) 



