PROCEEDINGS OF SECTION C. 395 



colorless and free from pleochroism, it is Jieither muscovite nor zim- 

 waldite, since it is distinctly uniaxial. It is like all tlie micas, negative 

 in character, and occurs in elongated irregular crystals and in matted 

 aggregates. 



The other mica is brown-green in color, and is probably a variety of 

 biotite, although not so strongly pleochroic. The brown-green mica 

 frequently occurs in radial groups of crystals, and the junction of the 

 two micas is generally sharp and distinct. In places along the cleavage 

 planes of the white mica the brown-green mica is developed. It is 

 possible that the white mica is bleached biotite, but the microscopic 

 evidence rather points to the possibility that the brown-green mica 

 has been formed after the white, as it is in places moulded on it, and 

 along cleavage planes the brown-green appears to be developed at the 

 expense of the white variety. A few crystals of tourmaline occur, 

 showing pleochroism from pale straw-yellow to a dark greenish-brown, 

 with occasional patches. The tourmaline sometimes encloses both 

 quartz and the white mica. Some grains of hematite and of magnetite 

 partially altered to hematite also occur. 



ROCK NEAR THE CONTACT WITH THE GRANITE, SECTION 354 

 (See Plate III., Fig. 1). 



The rock is an altered argillaceous sandstone, in which no trace of 

 bedding remains. 



The constituent minerals, in order of abundance, are tourmaline, 

 in irregular masses, which are yellow-brown to blue in color, and which 

 enclose quartz and white mica ; quartz, in interlocking crystals, some 

 of which have dusky centres ; white mica and brown-green biotite, 

 similar to those in No. 392 ; hematite, in red specks scattered through 

 the rock. 



No. :^87 (See Plate III., Fio. 2).— MOST ALTERED ROCK IN CONTACT 

 WITH THE GRANITE FROM A DITCH AT SOUTH END OF 

 PLANTATION, SOUTH-WEST CORNER OF STATION PEAK. 



Microscofic Examination.— TYio. granite consists of quartz, with 

 undulose extinction ; very kaolinised alkali felspar, white uniaxial 

 mica, and brown-green biotite. Since white mica does not occur in 

 the main granite mass, it is clearly a product of alteration of the felspar, 

 due to acid vapors and solutions at the margin of the granite — solutions 

 and acids which caused the tourmalinisation of the sediments, and 

 possibly the extreme kaolinisation of the felspar in the granite at the 

 margin. The junction of the white and the brown-green mica is quite 

 sharp. The white mica is possibly not bleached biotite, but may be a 

 new white uniaxial mica. 



METAMORPHIC PART OF SECTION 387. 

 The minerals and structure are similar to those in section 392, but 

 tourmaline and quartz are rather less abundant, while the white and 

 brown-green micas form the bulk of the rock. 



