J. FarsoHS — Brown Mica developed from Aurjite. 319 



(4) A gabbro from Humlebixk affords an interesting example of 

 the corrosion of a felspar by augitic magma, resulting in the 

 formation of brown mica. In this case the felspar is labradorite, 

 occurring in idiomorphic crystals, romid which the augite, a colour- 

 less variety, extends in ophitic fashion. The felspai's are much 

 corroded and in part melted down, thus enclosing small portions of 

 the augite. Round the edges of the felspai-, mica is developed, 

 appearing at the outset as a faint brown tinting of the augite. At 

 first the cleavage of this mineral is alone apparent as a rule, though 

 occasionally traces of the mica cleavage may be discovered parallel 

 to one of the cleavage planes of the augite. There are indications 

 that at this earliest stage a mixture of augite and mica is present, 

 or rather, a solution of mica in the augite is suggested. In one 

 instance where a felspar lath is surrounded by biotite, obviously 

 derived from augite, it has lost almost half its bulk by corrosion, 

 and the remainder appears to be permeated by augitic material, 

 converting it into a colourless mica whose cleavage planes are 

 continuous with those of the biotite on each side. The mica when 

 traced inwards from the margin of the augite loses its scaly 

 appearance, due to the cleavage of that mineral, becomes more 

 compact, and develops its own cleavage. At first it is a straw 

 colour, then it becomes a dark bright bi'own, and finally magnetite 

 is formed ; and in the larger augite grains this is followed by an 

 irregularly cracked zone which passes into a central portion showing 

 fine cleavage striation. With this, as a rule, the outer portion is in 

 optical continuity. When the augite is more remote from and 

 unaltered by the felspar, it is twinned sometimes more than once 

 parallel to the orthopinacoid ; this, combined with basal cleavage 

 striation, giving the well-known herring-bone structure. In the 

 smaller augite grains the conversion into mica and magnetite is 

 sometimes complete, a corona of light mica surrounding the bright 

 brown mica, and in most cases a core of magnetite occupying the 

 centre of the grain. The following explanation may be offered of 

 the transition shown in this rock and the syenites : — where the 

 felspar is in contact with the augite a mica is formed, poor in iron 

 and rich in alumina, probably phlogopite, while, as the distance 

 from the felspar increases, ferric oxide takes the place of alumina, 

 resulting in lepidomelane, biotite being formed as an intermediate 

 stage, and finally, beyond the limit to which the felspathic magma 

 permeated, magnetite is excreted. 



Thus the normal order of change seems to be as follows 

 (commencing from the side of the felspar) : (1) fusion of felspar 

 and augite with faint brown tinting of the augite, due to incipient 

 development of mica; (2) pale-brown mica (phlogopite); (3) 

 biotite ; (4) dark red-brown mica (lepidomelane) ; (5) irrtimate 

 mixture of dark mica and magnetite ; (6) mass of magnetite ; (7) 

 augite with irregular cracks in which magnetite is excreted, which 

 is followed by augite with fine cleavage striation. 



