86 GEOLOGY OF TRICHINOrOLYj &C. [ChAP. IV. 1. 



In plan the beds of rock composing- this hill show a very fine ex- 

 ample of a great sig-moid curve^ and the magnetic iron band ^ crops out 

 distinctly and continuously all round this curve. 



Again, Puggalavaudy hill (Great Trigonometrical-station) to the 



south of Toriore, when viewed from the west, shows 

 Puggalavaudy. 



the different beds dippmg south-east by south at 



an angle of 50°. Standing on any of the thick hornblendic beds half way 

 up this hill, one may trace its descent to the plain, where it runs along 

 eastward for miles, indicated by a dark row of angular debris between 

 similar rows of various width, but of lighter colour. 



Of the contortions of the gneissic rocks not immediately connected 

 with any one of the mountain ranges, the most remarkable example is 

 offered by the beds of the Agrarum Kotallum hill, in South Arcot. 



That these beds occupy their present position is evidently, in some 

 measure, due to their having been forced up into a broad-backed elevation, 

 the surface of which was then greatly denuded away, leaving the present 

 out- crops exposed to view. 



The series of beds here exposed consists chfefly of quartzo-horn- 

 blendic gneiss, with the important addition of a large and very conspicu- 

 ous bed of quartzo-felspathic gneiss (of pink colour generally), which may 

 be traced back to Ommacalatur, 6 miles south-west of the apex of the 

 curve at Kotallum. There is probably a connection -with a similar quartzo- 

 felspathic bed occurring still further to the south-west, near Karolanda 

 Kurchi, and it seems probable, if the beds could be perfectly exposed, 

 that the Ommacalatur felspathic gneiss would be found to be an exten- 

 sion of the identical rock which forms a conspicuous bed at the extreme 

 northern end of the Gungavully spur of the Patehamullays. 



If such should be the case, the Kotallum beds might fairly be con- 

 sidered as belonging to the TuUamullay-Kolymullay series of the great 



* In places round this hill the magnetic ii'on is converted into a latcritic rock for a 

 slight depth on the out-crop of this bed. 



■ ( 508 ) 



