1836.] 



hetween Madras and the Neilgherries. 



9 



Going towards the Fort or Pettah, a few yards before the cause-way, 

 we see immense masses of gneiss,on the convex surface of which are thick 

 veins of quartz and of felspar. In many of these veins, which contain 

 both these minerals intermixed, the surface is sometimes honey-combed 

 with numerous little cavities ; an appearance which is owing to the 

 decomposing and falling off of the felspar-pieces, leaving the places 

 they occupied empty. 



I must remark here a peculiarity in the structure of this gneiss ; viz. 

 that of splitting, both naturally and artificially, into laminae, the direc- 

 tion of which is nearly perpendicular to that of the seams of the strata,, 



In fact, in the laminae naturally detached from the rock, we observed 

 that the strata are seen either horizontal, or vertical on the surface of 

 them ; therefore the lamina? exfoliate in a direction at angles with 

 these strata. 



We see in all stratified rocks that they generally split in the direc- 

 tion of the strata; so that the surfaces of separation shew only the 

 surfaces of the seam. But, in the laminae of this gneiss, the case is dif- 

 ferent; on the surfaces of the laminae we see the strata, and their seams 

 along the surface of the split, and therefore its direction is at an angle 

 with that of the seams themselves. The annexed diagrams shew per- 

 haps better than any words what is meant in the above remarks 

 (Diagram, Nos. 1 and 2). 



It appears that the natives have availed themselves of the peculi- 

 arity this gneiss has, of splitting in a direction opposite to that of the 

 strata, to obtain lamina? of any thickness. The process to that effect is 

 very simple and economical. On the convex surface of the gneiss they 

 light afire, the intensity of which is proportionate to the thickness of 

 the slab to be obtained ; and, after having kept it up for such a length 

 of time, as experience has taught them necessary for the required 

 thickness, they extinguish it, and pour cold water on the heated sur- 

 face of the rock. 



This sudden refrigeration producing an instantaneous contraction of 

 the heated portion of the rock, extending as deep as the heat had pe- 

 netrated, it is detached at that depth from the parent rock, and the la- 

 mina is easily removed, and cut in as many pieces as required. 



The curvature of these laminae being the segment of a very large 

 circle, in the small dimensions they are generally cut they appear 

 nearly straight, and are used for all architectural purposes, as columns, 

 door-posts, steps, &c. 



I have read, I do not recollect where, that the foregoing process is had 

 recourse to, at Bangalore, to split granite. This must be amis-state- 

 ment, since at Bangalore, as well as in many other places in India, they 

 use another, and very different, method to split granite, porphyry, 

 green-stone, or other unstratifted rocks. 



