50 Transactions of the South African Philosophical Society. 



a light wine colour in the centre, showing feeble pleochroism with 

 greatest absorption along the trace of the short axis of the nicol. 



In this paste there are mineral aggregates, which appear to be the 

 shattered remains of a previously consolidated rock. 

 The included fragments are : — 



A little aggregate of orthoclase, plagioclase, biotite, sphene, mag- 

 netite, apatite, and chlorite ; the junction of the minerals is obscured 

 by an alteration product with the interference colours of calcite. 

 Little flecks of white mica are developing on the felspars besides 

 dusty alteration products. 



The orthoclase grains are in untwinned lumps showing shadowy 

 strain extinction, and also in little clear Carlsbad twins. The plagio- 

 clase is closely striated. The biotite is an irregular piece showing 

 the usual strong absorption when the cleavage lies parallel with the 

 short axis of the nicol. The sphene has a rim of titaniferous iron. 

 The chlorite is in a scale with very low bluish interference colours. 

 The felspar crystals show frayed ends where they have been partly 

 dissolved in the liquid magma, but along the sides the minute laths 

 of the matrix lie parallel to them without melting. As usual, there is 

 separation or segregation of magnetite near the ferro-magnesian 

 constituent. 



Another grain shows orthoclase, a slightly pleochroic augite, 

 sphene and magnetite. The felspars have a rim of absorption in 

 which, near the inner edge, there are little black tubules set at right 

 angles to the edge. Apatite occurs also in minute rods. 



Loose grains of orthoclase are found in the matrix showing some- 

 times clear rims of absorption. They are often associated with 

 biotite. 



Mica is common in detached flakes ; round them there is a zone 

 of clear crystalline substance full of magnetite granules. 



Sphene is very abundant showing a feeble pleochroism. The shape 

 of the crystals is characteristic, and the index of double refraction 

 and strong dispersion point to the identification of this mineral. 



Moderately pleochroic augites, yellow and greenish-blue, occur ; 

 they have not the absorption rims of the biotite, and seem to have 

 remained unaltered. 



Magnetite grains are rounded previous to being embedded ; the 

 felspar laths wrap round them, but there is no separation out of 

 magnetite granules as there is round the biotite. 



This extraordinary rock shows that the heat of its melting point 

 was sufficient to attack the ends of the orthoclase crystals, but not 

 sufficient to melt the sides ; it shows that it has the power of leach- 

 ing out the iron from the biotites but not that of the augites, and 



