302 On the formations of the 



nearly dry in a few days after having been filled; but the 

 change of level is so gradual, as to escape observation by the 

 eye. In proceeding west from Madras, by the valley of the 

 Palar, the plain of the Barramahal is gained at Vaniambady, 

 (which is elevated more than 2000 feet above the sea,) by 

 a gradual ascent the whole way, equal to about 10 feet in 

 a mile, or J-th part of the whole. 



528 



On examining the formation of these plains, by seeking 

 sections in nullahs, and by wells, it will be found that they 

 are composed of a series of thin contorted beds of crystalline 

 schists, porphyries, pegmatite, and trap, and generally the 

 absence of argillaceous slate and limestone, (as a rock,) is 

 deserving of remark. 



The rocks which compose this formation are the following : — 

 pegmatite, red or white felspar, porphyry, (as proposed in the 

 Madras Journal) ; I make use of the term porphyry in the 

 generic sense as defined by MacCulloch, and the rock to 

 which the above expression is applied, is formed of angular 

 crystals of red felspar embedded in a continuous mass of 

 white felspar, and in the following the preposition, " m" is 

 dropped. 



Also, quartz ; claystone ; quartz felspar ; porphyry ; trap ; 

 blacks chorl ; granular quartz and black mica in loose aggre- 

 gation, (the mica schist of some writers) ; mica slate ; horn- 

 blende slate, (rare) ; quartz rock, (very common) ; green- 

 stone sienite ; augitic sienite ; kunkur in- beds, or embedded in 

 and sometimes graduating into a rock resembling loosely 

 aggregated micaceous granite ; hornstone, (rare) ; friable 

 gneiss ; gneiss ; iron schist ; crystalline black hornblende. 



Some of the beds closely resemble granite, both micaceous 

 and hornblendic, in structure, composition, and general ap- 

 pearance ; but differ from the granitic formations in being 

 generally much softer, and frequently quite friable, and in 

 never shewing any signs of cleavage. 



