CiiAVTEli. IV. — Metamorphic roch. 



1. Gneissose Rocks. 



a. Varieties of gneiss. — The Metamorphic series, as developed in 

 the countiy here treated of, embraces a great succession of gneissose 

 rocks of various kinds, (chiefly of the hornblendic varieties,) and of 

 crystalline limestone in comparatively small quantity. This series con- 

 stitutes the bottom rocks of the country, no older 

 General character. . , . , . , . , 



formation having been discovered, and is overlaid 



successively by cretaceous rocks, the Cuddalore sandstones, and superficial 

 deposits of alluvium, &c. The different members of this series have 

 undergone great contortion in some districts, and have been broken 

 through by numerous granite veins, and traversed by dykes of greenstone 

 and other trappean rocks, and by veins of quartz. 



A very common and widely distributed form of these rocks is that 



of a dark grey, hard, compact, massive syenitoid 



gneiss, of quartz, hornblende, and felspar, in which 



the constituent minerals are pretty evenly distributed. It is this general 



type of rock which mainly constitutes the different mountain masses in 



our area. This is also the typical form of rock in the Nilgiris and in 



the Anamullays, two great hill-ranges which are beyond the boundaries 



of the district now referred to. Allied to this type, there is another in 



which hornblende occurs only to a very limited extent, being often almost 



Quartzo-felspathic ^^^irely absent. This is a massive quartzo-felspathic 



gneiss. gneiss, of a pale grey or buff color, and for the 



most part distinctly foliated. This variety is more particularly developed 



on the south side of the Cauvery, in the Trichinopoly district. 



Quartzites, or quartzose gneiss, occur only in one 



or two localities, as close to Naivailie. 



Hornblende schist alternates very frequently with the quartzo- 



Hornblendcschistsand felspathic variety of the gneiss. Hornblende occa- 



J'o'"^' . sionally predominates to such an extent as to con- 



(269) 



