PETROGRAPHICAL EVIDENCE IN SOUTH INDIA. 2lJ 



however, are points of evidence in favour of the igneous origin of 

 the rock-mass ; for the included fragment shows that the rock in 

 which it occurs was in a molten condition, whilst the schlieren 

 structures show that the magma must have been in a condition 

 of free molecular movement akin to that of molten material. 



When the production of schlieren results in the formation of well- 

 defined bodies included in the normal rock (as, for instance, the dark 

 patches so common in granites), such bodies might conveniently be 

 named autoliths in contradistinction to the term xenoliths applied 

 by Professor Sollas to picked-up fragments of foreign rocks {vide 

 infra, p. 234). Inclusions of rock similar, and perhaps related 

 genetically, to that in which they are included Lacroix proposes to 

 distinguish as homogeneous {enclaves homceo genes). x But the peculiar 

 meaning which we generally attach to the word homogeneous pre- 

 vents our adoption of Lacroix's expression ; the term homogeneous 

 applied to a body would to most people (following the usage of 

 mathematicians) imply similarity or uniformity throughout its own 

 parts, not similarity to its neighbours, or the matrix in which it is 

 embedded. 



In the charnockite series a common form of schliere (autolith) 

 appears as a dark-coloured, fine-grained, basic 



Basic Schlieren. ^ . 



fragment in the ordinary grey rock, which, on 

 microscopic examination, shows the same constituents as the main- 

 mass, but with a smaller proportion of the white (acid) constituents 

 quartz and felspar. An increase in the proportion of the peculiar 

 green-brown hornblende is an interesting feature very frequently 

 seen in these basic schlieren, interesting because a similar increase of 

 this hornblende always characterises the border facies of the basic 

 types in this series, the selvages of the Coorg dykes for instance 

 (p. 228). Like the border forms, these basic schlieren are thus pro- 

 ducts of the earlier stages of consolidation (see Nos. 1 1*910, 1 igi i, 

 11-917). 



1 Lacroix, " Les enclaves des roches volcaniques, 1893." 

 H2 ( 99 ) 



