PETROGRAPHICAL EVIDENCE IN SOUTH INDIA. 229 



shotted them to be finer in. grain, more basic and more hornblendic 

 than the central portions of the bands, and similar phenomena were 

 subsequently carefully examined and confirmed in other bands near 

 Somwarpet on the north-eastern fringe of the main mass of the 

 charnockite series (Nos. 12-395— 12-405). 



Although the selvages are not now glassy, or even felsitic, and 

 one would hardly expect them to be so after such an enormous 

 lapse of time, they are distinctly more compact and contain less of the 

 white constituents than the central portions of the bands. Like the 

 chilled selvages of ordinary dykes, too, the transition from the 

 compact to the ordinary form is very rapid, and there is no notice- 

 able difference between specimens taken a few inches from the 

 selvage and those taken near the centre of a 15-foot band. The in- 

 crease in the quantity of hornblende at the margins is a common 

 characteristic of the border facies of the norite family, with which 

 these dykes — as they must now be considered to be — are closely 

 related in chemical and mineral composition. 



Some dozens of these dykes may be counted between Somwarpet 

 and a few miles west of Fraserpet, and four or five of them, which are 

 sufficiently exposed, show the chilled selvages most distinctly. Those 

 which are exposed so well in the bed of the Cauvery river at Fraserpet 

 are found to vary considerably in thickness as they are traced in the 

 south-east direction across the river, but no constant average diminu- 

 tion in either the north-west or south-east direction could be definitely 

 determined in the short distance of about 5 mile for which they are 

 exposed in the river bed. One of them, which measured 2 feet 8 

 inches in width on the left bank, diminished rapidly near the 

 centre of the river to less than a foot, but as quickly widened out 

 again before the right bank was reached. Similar, but not constant, 

 variations were noticed in the larger parallel dykes. From the size of 

 the small one just referred to these dykes may vary up to 100 yards 

 or more in width. 



Garnets are found in most of them ; sometimes concentrated in 

 patches of indefinite outlines, at other times arranged along lines 



( in ) 



