J. Parsons — Brown Mica developed from Au/jife. 31 7' 



of Soutbern India. In this instance, after augite and plagioclase had 

 consolidated a hydrous magma was left, which ultimately solidified 

 as micropegmatite ; this magma acted readily on the augite, con- 

 verting it into biotite.' Under this bead the following observations 

 on some Norwegian rocks should px'obably be classed.^ 



(1) A granite from Almiis shows a similar process of change to 

 that observed in the above-mentioned augite-diorites. The principal 

 constituents of the rock are, oligoclase, orthoclase, augite of the 

 pale-green non-aluminous type (sahlite), which commonly occurs in 

 granite, biotite, and sphene. The biotite is developed round the 

 augite, which is much corroded, passing into it along its cleavage 

 cracks, in such a way that, in many cases, the line of division is 

 fixed with difficulty. The two minerals are curiously intergrown, 

 but not in such a way as to suggest simultaneous crystallization, for 

 while the biotite invades the augite from the outside it does not 

 form isolated inclusions in the latter. The quartz and orthoclase 

 occur as a mosaic of varying coarseness and of later date than the 

 oligoclase, and locally form patches of micropegmatite. The mosaic 

 extends in narrow seams between the oligoclase crystals, broadening 

 out at their angles to fill the intercrystalline spaces. It is largely 

 developed round the augite-biotite aggregate, the grains separating 

 the flakes of biotite at the margins of tlie main masses. Appearances 

 suggest that the magma, from which the quartz and orthoclase 

 formed, efiected the corrosion of the augite and subsequent develop- 

 ment of the biotite in the manner suggested by Mr. Holland. The 

 sphenes occur as idiomorphic crystals, and are the earliest con- 

 stituents of the rock, for they are included in the augite, which 

 is itself earlier than the oligoclase. Where present in the biotite 

 or in its immediate neighbourhood they are much corroded and 

 cracked. The iron of the augite appears to be completely used up 

 in forming the biotite, for though a few grains of magnetite occur, 

 these are clue to the subsequent hydration of the mica accompanied 

 by the formation of a chloritic mineral. A little hornblende is 

 present, apparently secondary, after augite. 



(2) An augite-syenite from Svenore well shows the conversion 

 of augite into dark mica. This rock is closely allied to the well- 

 known Lauvigite. The felspars are orthoclase, apparently of two 

 varieties, with a little plagioclase. They exhibit microperthitic 

 intergrowth, and this in many places at their margins passes into 

 micropegmatite. The augite is of two types : (i) Pale brown, slightly 

 pleochroic, showing, in the central portions of the crystals, good 

 cleavage marked by the deposition of iron oxide in the cracks, which 

 occasionally assumes the form of negative crystals. Incipient 

 diallagic cleavage may be observed, traversing the earlier prismatic 

 one. This augite is much corroded by the felspars, its margin 

 being in some places curiously broken up and intergrown with the 



1 Q.J.G.S., vol. liii, p. 405. 



- I am indebted to Professor Bonuey for the o])portuiiity of studying these slices, 

 which belon"- to his collection. 



