PETROGRAPHICAL EVIDENCE IN SOUTH INDIA. 21$ 



oscillations of level may give us rapid alternations of shales, sandstones 

 conglomerates, marls and carbonaceous deposits, and no amount of 

 metamorphism short of complete fusion could ever produce uniform- 

 ity of composition throughout such a complex formation of beds. 



In old metamorphic rocks like the Dharwars and so-called upper 

 division of the schists, we get beds of quartzites, hornblende-schists, 

 mica-schists, iron-ore beds, conglomeratic schists and crystalline 

 limestones — rocks which form comparatively narrow bands wholly 

 distinct from one another and clearly diverse in origin and in age. 

 There is no equivalent amongst formations like these for a homogene- 

 ous mass of rock measuring 15, 20 or 30 miles across the direction of 

 foliation. To regard the enormous thickness of the charnockite series 

 as the result of repeated foldings of one formation would be no help 

 out of the difficulty, but on the contrary would leave us with the still 

 more difficult task of finding the evidences of folding. Traverses 

 across the foliation lines reveal no regularly repeated succession in 

 composition and structure ; for the foliation is generally a micros- 

 copic structure and the banding seldom continuous for more than a 

 few inches {infra, p. 221). 



But whilst, like pluton ic bosses, the masses of these rocks are 

 uniform in their general average characters 



Schlieren structures. , . , , . 



over large areas, they are seen on close examin- 

 ation to present precisely the heterogeneity of structure and 

 composition which is characteristic of igneous masses — masses in 

 which there has been sufficient freedom of molecular movement to 

 permit local differentiation of the compounds, or segregative consoli- 

 dation of the mineral constituents. 



Such local departures from the average composition of the rock 

 are spoken of by German geologists as Schlieren, a term which for 

 want of an exact equivalent we might profitably borrow. Because 

 the word Schliere merely indicates a structural phenomenon without 

 regard to the ultimate cause of its origin, it is likely to convey only its 

 structural meaning, and is therefore preferable to the equivalent ex- 

 pressions segregations and concretions which are used in various 

 H ( 97 ) 



