234 NILGHIRI HILLS. 



containing a few crystals of Pyrites of tolerable size, and which appear 



to have the form of the pentagonal dodecahedron. 

 Pyrites. 



In the vein which is seen cropping out on the 



hill side where the Avalanche road crosses a stream about 7 miles from 

 Brown Hcematite how Ootakamund, there is a small quantity of Brown 

 ^''""''- Haematite (Limouite) filling the cavities in the 



quartz. This is sometimes seen forming irregular pseudomorphs of 

 the Pyrites, and it is evident that it has resulted from the de- 

 composition of that mineral. Captain Ouchterlony mentions having 



found a speck of copper Pyrites in a loose block 

 No Copper Pyrites. i • i /. 



derived from one of these quartz veins, but no 



trace of that mineral has been found by the Surveyors after careful 

 and repeated examination of the veinstones. It is possible the Iron 

 Pyrites, or Mundic above described, may have been mistaken for the 

 more valuable cupriferous mineral. 



Weathering of Rocks. — Before proceeding to describe the series of 

 Weathering prineipal- V'^Y^^cal changes by which the hiUs have been 

 ly a chemical question. denuded to their present form, a few words must 

 be said on the decomposition of the rocks. There are many points 

 connected with this subject of a purely chemical character, and which, 

 although of great geological importance, can only be worked out in the 

 laboratory. Thus, without a carefully conducted chemical analysis, it 

 is difficult to assign a reason for the unequal and irregular decomposition 

 which obtains in many cases in rocks of apparently identical mineral 

 composition. This appears indeed occasionally to be due to the 

 varying relative amount of felspar present, and 



Dependant on the 



amount of felspar and probably also to local variations in the composi- 

 hornblende present. 



tion of the felspar, but what these differences are, 



and in what manner they affect the stability of the rock, chemical 



examination alone can determine. As a general rule, those rocks from 



which felspar is absent, or in which it is only present in small quantity, 



