CLASSIFICATION OF THE ROCKS. 109 



a close resemblance in peculiar points, and, taken into con- 

 junction with the fact that the gneisses present in all their 

 occurrences essentially similar geological relations to the associated 

 pyroxenic rocks, the evidence as to their identity is as satis- 

 factory as can be expected in purely petrological correlations. The 

 two chief peculiar points in common throughout these occur- 

 rences are the abundance of the highly doubly refracting needles 

 and spindles in the felspars, and the bunchy grouping of the 

 biotite, with its tendency to take on a green colour by passage 

 into chlorite. These are both features of secondary origin, of 

 course, but they are essential to show that the gneisses, besides 

 having the same primary composition, have passed through similar 

 secondary changes, which, within a limited area of only about 5 miles 

 radius, ought to be sufficient to warrant their being mapped as isola- 

 ted exposures of the same formation. The points of difference 

 between the specimens are merely due to variation in size of grain 

 and degree of alteration. 



It is interesting to note that the features, which characterise 



these old gneisses around Salem, are repeated in 



Ident ron^meratoS. arwar a ver T striking manner in the gneiss pebbles of 



one of the schistose conglomerates of the 

 Dharwar rocks in the adjoining State of Mysore. A section of one 

 of these pebbles, kindly presented to the department by Dr. J. W. 

 Evans, late State Geologist of Mysore, is so exactly similar to some 

 of the Salem biotite-gneisses that it might almost have been cut 

 from the same specimen, the differences between the Dharwar pebble 

 and, for instance, the gneiss west-south-west of Salem (No. 11 892) 

 being less than the variations which are presented in different parts 

 of the area now under discussion. Another pebble from the Dharwar 

 conglomerate is also derived from this old type of gneiss, though not 

 so strikingly similar to the rocks in the vicinity of Salem. 



It is not intended by this comparison to imply that the gneisses 

 around Salem formed the source of the Dharwar pebbles ; for the old 



( 7 ) 



