DESCRIPTION OF THE PRINCIPAL EXPOSURES. 187 



such as occur, for instance, in the river-bed below the Coonoor 

 bridge. 



The inclusions referred to are formed of extremely tough 

 rocks, with a specific gravity varying between 3*13 and 3*25. They 

 often exhibit a remarkable lustre-mottling due to the development of 

 pyroxene crystals measuring sometimes six inches across. Under 

 the microscope thin sections of the pyroxene are quite colourless 

 and appear to be changing into a peculiar pale-yellow hornblende, 

 Eyes of clear plagioclase occur enclosed in the pyroxene, and as 

 several apparently isolated sections of this plagioclase show simul- 

 taneous extinction, they are probably in crystallographic continuity 

 with one another (No. 9302). The structure is, therefore, due, not to 

 ophitic development of the pyroxene, but to a pegmatoidal intergrowth 

 of the two minerals. Sometimes the pegmatoidal structure is 

 destroyed and the rock becomes granulitic (No. 8*760). Much of 

 the pyroxene is rhombic and in thick sections shows a very faint 

 pleochroism. The hornblende shows its characteristic prismatic 

 cleavage which facilitates the determination of its optical properties. 

 The extinction angle is 18 (c:c). The pleochroism is well 

 marked though not so strong as in common hornblende — 

 a = very pale yellow. 

 fo= brown-yel low. 

 c=yellow. 

 The optical properties of this mineral agree] with those of the 

 3rd variety of hornblende in Lacroix's " pyroxenic gneiss b " (Rec. 

 Geo/. Surv. Ind., Vol. XXIV, page 182), which was likewise found in 

 a rock with pegmatoidal pyroxene and felspar. Unfortunately, how- 

 ever, Lacroix has given no hint as to the geological relations of the 

 rocks described, so this partial agreement in microscopic characters 

 might be purely fortuitous. 



The biotite which invariably occurs in these inclusions is a 

 highly pleochroic variety varying from deep yellow-brown to very 

 pale yellow. Flakes examined in convergent polarized light show 

 a narrow optic axial angle with a negative bisectrix. Neither free 



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