112 GEOLOGY OF TRICHINOPOLY, &C. [ChAP. V. 



of a rather coarse-grained, but exceedingly hard and tough, black 

 basalt-lilce mass, hardly ever containing recognizable crystals of any 

 foreign substance. 



In a few cases the fresh fracture showed a greyish lustre, due appa- 

 rently to numerous small crystals of a felspathic mineral. 



In the case of a dyke occurring close to the village of Agaram 

 Porplm-itic trap of ^^o^^-Hum, north of the Kotallum hill station in South 

 ^^' Arcot, the black rock contains numerous crystals 



of a glassy variety of felspar, resembling Labradorite in textui'e and 

 lustre, but without any iridescence. These are probably a variety of 

 Sanidin. The crystals obtain a size of as much as 2 to 3 inches in 

 length, by from 1 to 3 in width, but are generally much smaller. The 

 matrix is very black and exceedingly hard, so much so that even 

 a heavy hammer failed to make any impression on the spherically- 

 weathered masses. This dyke is but of small extent, and none of the other 

 trappean intrusions in the neighbourhood exhibit these crystals and 

 the resultant porphyritic character. The course of this dyke is from 

 south-west by west to north-east by east, and it gives off a branch of 

 equal size with itself with a north-easterly course. 



One of the most striking features in the rocks being the almost 

 total absence of olivine and titaniferous magnetic iron, which minerals 

 are so characteristic of the generally more common and typical variety of 

 basalt, there can be little doubt that most of these dykes may be 

 considered as of Anamesite. 



Olivine was met with unmistakeably in only two of the many 

 score of dykes examined, namely, at the Pulioor trap-dyke before named, 

 and in a large dyke south-east of Vettratty, in the Tummumputty 

 valley. 



Of the age of the trap-dykes, we know only that they are pre-cre- 

 ,, , . . n , , taceous. The evidence of this is found in the 



Geological age ot the 



^^'^'^^- case of the dykes at Puravoy, above alluded to^ and 



( 334 ) 



