Chap. IV. 1.] metamorphic rocks. 83 



an area several huudred square miles in extent. Those met with also are 

 all rather small in size and of little length. They do not, however, pre- 

 sent any peculiarities of structure or of mineral contents. 



Details of Metamorphic country. — Over the whole area at present sur- 

 veyed, the lamination of the metamorphic rocks 

 is distinctly visible, although more decidedly in 

 the magnetic iron beds than in the hornblende rocks and schists. The 

 general direction of this foliated structure is, for the southern and cen- 

 tral parts of the area, east-north-east and west-south-west, but in 

 the northern part it trends to the north-east, north-east by north, 

 and even north-north-east. Great and frequent alterations are, how- 

 ever, as might be expected, observable in the neighbourhood of igneous 

 rocks, as along the south bank of the Cauvery, and around and north- 

 west of TogamuUay, and in various parts of the country where the 

 strata give evidence of their having undergone immense pressure and 

 consequently extensive disturbance and distortion. Local changes in 

 direction over a small area are very frequently met with, but the original 

 direction is almost immediately re-assumed. Near the village of Pooja- 

 riputty (13 miles north-west of Trichinopoly) there is a boss of gneiss, 

 which in plan sho'ws a sigmoid contortion in the foliation.* But the beds 

 of which it is composed almost immediately resume their normal strike of 

 east and west at each extremity of the curve. Again 16 miles south- 

 south-east of Caroor, an exactly similar phenomenon is observable in 

 hornblendie gneiss, of the same character as that of Poojaripulty, while 



* By the term foliation we intend to imply such, an arrangement of the constituent 

 minerals entering into the composition of the gneiss as gives to the mass of the rock the 

 more or less streaked appearance which they present. At the same time all oiu- observa- 

 tions have tended to show that, within the area described, the planes of such folia are 

 parallel to those planes obviously resulting fi-om original deposition, and therefore that 

 foliation here coincides, in direction, with stratification. On the exposed and Aveatliered 

 masses of rocks this foliation sometimes shows itself merely by lines of different colours on a 

 tolerably smooth and even surface, as is generally the case in the quartzo-felspathic gneiss. 

 In other cases tliis foliation has given rise to narrow and slight furrows on the surface, when 

 the more easily decomposed minerals of some of tlie folia have been weathered out to 

 a certain depth. 



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