EL/E0L1TE-SYENITE. I95 



The smaller size and lesser quantity of the elaeolite are 



noticeable on the weathered surfaces of this rock, very 



few of the small pits being detected. 



This rock forms large masses on the western face of Siva- 



malai ; but its precise relations to the other rocks could not be made 



out. The dark hornblendic pitches are too evenly distributed to 



suggest a mixing of two types, though it is not unlikely that this is a 



stage in the aggregation of the hornblendic constituents, which in 



other parts of the hill form large, black lenses (le). Like all the 



other rocks of Sivamalai this variety is roughly foliated. 



I have more than once had occasion to refer to the peculiar 

 tendency which granulitic rocks of this kind show towards an aggre- 

 gation of constituents into groups of like kind. It is quite a com- 

 mon feature of the charnockite series, especially the intermediate 

 varieties, 1 and is due in my opinion to movement of the magma 

 during the process of consolidation, whereby the crystallizations 

 around definite centres are disturbed, and what would have been a 

 crystal of large size under quiet conditions is converted into an 

 aggregate of minute granules. For this structure Loewinson-Les- 

 sing has recently proposed the name glomero-plasmatic structure as 

 an analogue of Judd's glomero-porphyritic structure. 2 The structure 

 is illustrated in Fig. 3, which represents the natural size of a piece 



1 Mem. Geol.Surv. Ind., XXVIII, pp. 152, 241. 



2 F. Loewinson-Lessing. " Geologische Ski?ze der Besitzung Jushno- 

 Saosersk und des Berges Deneshkin Kamen im nordl. Ural", Jurjew (Dorpat) 

 1900, p. 208. Describing a hypersthene-gabbro-norite of the Deneshkin Kamen, 

 Loewinson-Lessing says, " lin rosenfarbener deutlich pleochroitischer Hyper then 

 kommt ausschliesslich in abgerundeten Kornern vor, die zum Theil vereinzelt 

 im Gestein verstreut, theils in Aggregate gruppiert sind, wodurch das Gestein 

 in ganz eigenartiges Geprage enthalt. Solcheine Structur, welche durch die 

 Gruppierung eines oder mehrerer Gemengtheile in kornige Aggregate gekenn- 

 zeichnet wird, die in eine kornige Masse von anderer Zusammensetzung einge- 

 schlossen sind, weist eine grosse Analogie mit der glomeroporphyrischen Structur 

 auf und konnte als glomeroplasmatische Structur bezeichnet werden. Im vor- 

 liegenden Fall sehen wir kornige Hypersthen-aggregate, die als dunkle Flecken 

 im Fond eines normalen leukokraten kornigen Gabbronorits^hervortreten. 



( 27 ) 



