CllAV. y.J CRYSTALLINE HOCKS. Ill 



The two nortliern members of this series arc very fine trap-dykes^ 

 descending from the northern end of the Tainauda- 



Huroor dykes. 



mullay^ crossing- the Carapaudy ridge^ and descend- 

 ing thence in two very conspicuous spurSj they run north and south of 

 the Huroor hill^ and may be traced with a few breaks from Huroor, till 

 they cross the limits of the map after courses of 27 or 28 miles, remark- 

 able for their close parallelism. How great the westward extension of 

 these dykes may be, remains yet to be ascertained, and their extreme 

 eastern termination may, in the case of the three more southerly members 

 of the series, be found hidden in the dense forests of the TainandamuUay 

 a mile or two east of the spots they were traced to at the time of the survey. 

 It will be seen that the distribution of the dykes is very irregular ; 

 Absence of dykes south ^^ne of any size were met with south of the 

 auvery. granite region on the north bank of the Cauvery, 



while all over the country occupied by the ^' granitoid gneiss,^' the trap- 

 dykes are few in number and small in size. 



Eastward of the granitoid gneiss region they again appear of larger 

 size, as, for example, the two very remarkable 

 dykes running from Semangalam, 12 miles north- 

 west of Pondicherry, through the Cunum and Perumbakkam hill stations 

 to the Trichinopoly and Madras road. Of all the dykes met with, none 

 present so remarkable a case of parallelism as these two ; when seen from 

 the village of Cunum, they present a very remarkable appearance, not 

 easy to be described, but suggestive of two rather crowded rows of ruined 

 " martello towers'^ of black colour, the ridges having been eroded into an 

 avenue of rather tower-like black bosses. 



Several of these bosses cannot be much under 100 feet in height 

 above the very flat country at foot of them. 



The general mineral character of by far the greater number of the 



Mineral character of trap-dykcs scattered SO profusely in various parts 



^ *'^' of our area, is identical; they consist generally 



( 333 ) 



