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ARCH^AN AND PLUTONIC ROCKS. 27 



The rocks met with over the Archaean area show considerable 

 Division of the Ar- variety in their composition and structure 

 chsean rocks. (texture), as might be expected, and they may 



be divided accordingly in the first place into granitoid and gneissic, 

 and again, in the second place, according to their mineral composi- 

 tion, into the following groups : — 



a. Orthoclase, quartz, hornblende. — Granitoid, and mostly strongly- 

 porphyritic (Hornblendic granite). — Colour, dark grey or purplish 

 pink, weathering reddish or brownish. 

 So r* Q uartz i orthoclase, hornblende.— (Quartz Syenite), rarely por- 

 < <Z h " < phyritic ; colour, silvery grey to dark grey ; weathering grey or 



whitish ; mica occasionally as an accessory mineral. 



c. Orthoclase, quartz, hornblende, or mica, rarely visible; probably 

 younger than " a " and u b ;" often very pegmatoid in appearance. 

 Colour, pale grey? : weathering pale, dirty pink. 1 



d. Quartz, orthoclase. — Hornblende : gneissic in texture or semi-grani- 

 toid and banded. 



e. Quartz, orthoclase. — Mica : gneissic in texture or semi-granitoid and 

 banded. 



Both members of the gneissic division appear younger than "a " 

 and " b" of the granitoids; their relations to"c n have not been 

 ascertained, and they have not been noted in juxtaposition inter se. 

 The inter-relations of "a " and <; b " have yet to be established, no 

 section having been met with which showed them in contact. 



No attempt is made to show the different varieties of granites and 

 Why not mapped gneisses in the map ; they are too much mixed 

 separate y. U p - n nature to a d m it f being mapped sepa- 



rately, except after a much more exhaustive survey on much larger 

 scaled maps than I had the means or the time for making. 



The view which was some years ago so very prevalent among geo- 

 logists that granitoid rocks, where met with in 

 Past and present & & 



views concerning the great mass,must be regarded as of originally sedi- 



crystalline rocks. . . . 



mentary origin and converted into their present 

 crystalline conditions by vast metamorphic action, appears to me, after 

 long study in the field (largely in the Bellary region), to be quite 

 untenable. The rocks which I had learnt to regard as derivative 



1 This granitoid is always deeply weathered and, being of no value as a building 

 material, is hardly ever broken into deeply enough to show its proper olour. 



( 27 ) 



