166 FOOTE : GEOLOGY OF THE BELLARY DISTRICT. 



mostly composed of a hornblendic diorite, but in some few cases the 

 material composing them was submitted to microscopic examina- 

 tion by my colleague Mr. Philip Lake and found to be mainly 

 augitic instead of hornblendic. It is clear that slices of the 

 rock of all the great dykes deserve examination to determine their 

 composition accurately, but this is a task I have been unable to 

 accomplish from want of time and skill in preparing sections, and 

 no petrographical assistance has been procurable from the Survey 

 Office in Calcutta. The establishment of a special petrological 

 branch in connection with the Survey is a very great desideratum, 

 and must be established soon if the Department is to maintain its 

 position as a scientific one and not to degenerate into a mere 

 mining record office— a fate which now seems to be in store for it at 

 no distant date. 



If the dykes be grouped according to the direction in which they 



Grouping of dykes run ; J 4 groups will have to be established which 

 by their courses. groups, however, will be of very unequal import- 



ance numerically. The courses of few of the dykes agree exactly 

 with any of the points of the compass and not many are truly 

 parallel to each other, so the groups can only be formed by including 

 in them dykes running in approximately similar directions, each dyke 

 therefore must have its course lying within 5 37' 30" of the point 

 forming its group index. 



The 260 dykes met with in the several areas, into which the 



Distribution of the Archaean portion of the district has been divid- 

 dykes ' ed for convenience of description (see p. 27) — 



are unequally scattered about, being numerous in some parts and 

 but sparsely distributed in others. On the whole, the Bellary district 

 can boast a fair number of dykes, but it does not approach in rich- 

 ness in these intrusive rocks the tract lying eastward of it from the 

 Tungabhadra southward and including the crystalline areas in 

 Karnul, Anantapur, North Arcot and North-Eastern Mysore in which 

 the intrusion has taken place on a most extraordinary scale both for 

 the size and number of dykes. 



( 166 > 



