THE METAMORPHIC ROCK8 OF ADBLIE LAND. 8TILLWELL. 33 



No. 412. In this case the hornblende and biotite exist in practically equal 

 proportions. There is a well-developed schistosity, and the mica flakes give a bright 

 sheen to the cleavage surface. The grain size is about normal and a little larger than 

 that of No. 153. In thin section (Plate I., fig. 2) the hornblende and biotite possess the 

 usual characteristics. The greater proportion of the felspar is quite clear and transparent, 

 the lesser portion is saussuritised as in No. 5. Calcite is present in coarse granular 

 crystals, and has probably developed in the alteration of the felspar. Epidote is very 

 distinct in pleochroic crystals and may be intergrown with the biotite or with the 

 hornblende. In both cases it may exert its crystal form against the hornblende and 

 the biotite. A small percentage of lawsonite is recorded in the rock, and it is usually 

 interlaminated between the cleavage veins of the biotite. It might be mistaken for 

 colourless epidote, but a difference is obvious when the two minerals are brought into 

 the same field of view. Lawsonite attains its best development in Nos. 720 and 635, 

 and its characters will be fully defined from those sections. Occasionally purple 

 fluorite is threaded with the biotite in the same manner as in No. 153. Sphene is 

 practically absent, and there are only very few particles of iron ore and crystals of 

 apatite. No. 412 may be called a biotite amphibolite schist. 



No. 720. The example is a lustrous schistose rock in which hornblende, mica, and 

 some colourless felspar or lawsonite are visible to the naked eye. The mica has a golden- 

 brown colour. 



In thin section (Plate I., fig. 6) the hornblende has its normal appearance, but the 

 mica content consists of intergrown biotite and chlorite in which the latter is dominant. 

 The biotite tends towards a biscuit-brown colour that is best developed in No. 635, 

 and it is pleochroic from this brown colour almost to colourless. The chlorite is also 

 pleochroic through shades of pale green to colourless. Hence, in some positions of 

 the polariser, the intergrowth of biotite and chlorite may look homogeneous. The 

 composite character is readily observed by rotating the stage or by crossing the nicols. 

 The chlorite possesses bright ultra blue polarisation colours, and the biotite shows 

 brilliant second and third order colours. The " felspar " percentage consists entirely 

 of cloudy brightly polarising aggregates. Clear felspar is absent. The association of 

 saussuritised felspar and lawsonite seems to be very characteristic. The lawsonite is 

 abundant and may be intergrown with the mica, less frequently with hornblende, or 

 it may appear in laths with parallel arrangement contributing to the schistosity of the 

 rock. When best developed the laths are clear and colourless with prominent parallel 

 cleavage. Sometimes it is a little cloudy and some lawsonite areas enclose pieces of 

 saussurite, thereby indicating the connection. The lobate outline is conspicuous when 

 intergrown with biotite, but the laths mostly possess straight sides when intergrown 

 with hornblende. A slight lobateness is sometimes detected against the hornblende, 

 but is never prominent. A section has been noted where the end portions of a lawsonite 

 crystal are bounded by hornblende crystals, while the middle third abuts against chlorite. 

 The junction against the hornblende is linear at both ends, but the middle portion is 



Seriei A, VoL m.. Part 1 C 



