Ghap. v.] crystalline "rocks. 109 



to two epochs, but, on the contrary, there is a simple junction of homo- 

 geneous rocks, or, if there be any difference in the texture of the trap, it 

 consists only in the rock composing the smaller branches and dykes being 

 rather more compact and fine-grained than in the larger ones. 



Westward of this system of dykes, we observed a remarkable dyke 



stretching in an almost perfectly straight line from 

 Vailoocoorchy dyke. 



the ridge of the Neinahmullay (8 miles north- 

 north-east of Namcul-droog) in a north-east by north direction for 12 

 miles. It is not unlikely that it may be continuous with a large dyke 

 running from the east flank of the Anandymullay, near Jyelputty, to 

 within a mile of the tenth mile-stone east of Salem, on the Cuddalore 

 road. The break of 4 miles between the two dykes, as shown on the 

 map, would probably be filled up, could the surface soil and jungle be 

 removed from the flanks of the Anandymullay. 



This dyke is remarkable because it appears to be connected with a 

 Dislocation of mag- ^^®^* dislocation of the gneissic strata which it 

 netic iron beds. crosses. This dislocation, which has already been 



alluded to in the Chapter on the magnetic iron beds (page 64), would ap- 

 pear to consist of the shifting of the beds forming the great magnetite 

 ridge of Vailoocoorchy by a fault, the downthrow being to the northward. 



The fault appears along the outline of a curve in the bedding, 

 by which the amount of downthrow appears to be enormously exag- 

 gerated. 



The evidence in favor of this downthrow consists in the absence of 

 iron beds on the north side of Vailoocoorchy village corresponding with 

 those on the ridge north-west of that place, and in the diminution of the 

 angle of the beds on the east side of the fault to 72° south from a dip of 

 85° south, at the east end of the ridge north-west of Vailoocoorchy. 

 "VVTiether or not the trap-dyke was formed at the same time as this line of 

 fracture, or the trappean rock subsequently injected, there is no evidence 

 to decide. 



( 331 ) 



