88 GEOLOGY OV TRTCMTXOl'OLY^ &C. [ClfAP. IV. 1. 



quartzose gneisSj and this borders the great band of massive syenitoid 

 gneiss of the Kolymullays and Patchamullays. 



Neglecting the slight changes in the direction of the dip of the bed- 

 ding which have been observed in those localities where the intrusion of 

 igneous matter has taken place, there is a very decided connection between 

 the inclination of the beds and these different bands of rock described above. 

 North of the Cauvery and nearly u.p to the foot of the Kolymullays the 

 rocks have a general dip northwards, while about 6 miles south of the 

 river, where the corresponding band of hornblendic rocks comes in, the dip 

 is reversed, and the beds which lie between that and the northern part of 

 Madura district dip regularly to the south. The region of quartzo-fels- 

 pathic gneiss lying between these two bands of the hornblendic series 



shows the same change in dip on its two edges, thus 



Great anticlinal fold. . . ... ^ o t ^ n ^ • ^ n • -i 



indicating a great antichnal told, ot which this mid- 

 dle band is the nucleus. The axis of the fold runs with very little variation 

 in an east-north-east direction, and may be traced from Vellyana (8 miles 

 south-south-east of Caroor) by Pereya Sainkul and across Retnagherry 

 hill station, beyond which it is lost in the granitic district to the east. 

 A great fault, with a downthrow to the north, has also apparently been in- 

 duced along the northern side of this axis, or rather along the band of 

 ioneous intrusion, as the hornblendic series of rocks lies much nearer to 

 the axis on the northern side than on the southern. 



Around Trichinopoly the country does not present so uniform a 

 character in the dip of its rock beds, Avhile immediately south of the 

 Patchamullays and in the South Arcot low country round the Coortalum 

 Trigonometrical Station hill, as well as west of Tullamullay and the 

 Kolymullays, the out-crops of strata traverse the country in g'reat curves, 

 which are distinctly traceable for many miles. 



The physical features of the country in which the two divisions of 

 gneiss are described as occurring'^ differ considerabl}'. 



Physical features. 



The peculiar rounded bosses and hills SO common on 

 the banks of the Cauveiy, and south of it, e. g., Trichinopoly rock, 



( 310 ) 



