ORIGIN Of THE CORUNDUM. 405 



IV.— ORIGIN OF THE CORUNDUM. 



The corundum occurs in the felspar-rock as a normal, primary 

 constituent, the crystals being idiomorphic in outline and embedded 

 in the felspathic material without a recognisable " court" or peri- 

 pheral alteration. The crystals are sometimes greenish-grey, some- 

 times blue and variegated, showing a tendency to the development of 

 a tabular habit, which Lagorio considered to be characteristic of 

 pyrogenetic corundum, and which is the habit of crystals obtained 

 by the crystallization of artificial slags. 



The villagers generally select the foot of each small hillock of 

 elaeolite-syenite for their prospecting operations, which indicates 

 that the corundum is most abundant near the junction of the felspar- 

 rock and the elaeolite-syenite. But as the ground around is cultivated 

 and the rocks entirely hidden, it may be also that the felspar-rock 

 itself is limited to the immediate neighbourhood of the elaeolite- 

 syenite lenses. Where the two rocks are seen in actual contact 

 there is no chilling on either side, and nothing to show secondary 

 contact effects ; it is highly likely in fact that their intimate associa- 

 tion is the result of a common origin* 



This view is rendered all the more plausible by the fact that 

 similar associations of elaeolite-syenite with a corundiferous syenite 

 were discovered independently, and, curiously enough almost simul- 

 taneously, in other parts of the world, namely, in Ontario and in the 

 Ural mountains. Taken in conjunction with the facts obtainable from 

 Sivamalai, these occurrences appear to suggest a simple explanation 

 for the fact that the corundum is limited to the felspar-rock and is 

 not found in the aluminous elaeolite-syenite. 



Descriptions of the Canadian occurrences had not been published 

 when I first announced, early in 1898, 1 the occurrence at Sivamalai, 



1 Economic Geology of India, 2nd Ed., Part I, Corundum, 1898, pp. 11 and 37. 



( 37 ) 



