TAWNG-PENG SYSTEM : INTRUSIVE ROCKS. 



61 



the felspars (Plate 8, figs. 1, 2). This rim consists, as in the S. 

 Rewa rock, of two zones, the inner very narrow, composed ot 

 a colourless minutely granular mineral, while the outer zone, which 

 is often much broader, consists of a finely fibrous, also colourless 

 mineral, which may be either actinolite or tremolite. In the S. 

 Rewa rock the inner zone was considered by Sir T. Holland to be 

 enstatite, on account of its optical continuity with the adjoining 

 crystals of that mineral, but the Nam Hsan rock does not con- 

 tain any enstatite, and it cannot therefore be determined to what 

 species the mineral of the inner zone must be referred. The 

 " reaction rim " is entirely absent where the olivine comes into 

 contact with the augite, but it sometimes expands into the 

 interstices between the other constituents of the rock. A little 

 mica is sometimes included in it. The peculiar colour of the 

 olivine, which is perhaps more pronounced in the Nam Hsan 

 rock than in that from S. Rewa, is seen, under a high power, to 

 be due to inclusions of dusty matter, some of which may be 

 manganese, for Dr. Fermor informs me that a trace of this mineral 

 occurs in the Nam Hsan rock, and the analysis given on p. 20 

 of Sir T. Holland's paper shows that it occurs in his specimen 

 also. 



The felspar is a variety giving a fairly high angle of extinction, 

 either a basic andesine or labradorite. It possesses a light reddish 

 brown colour, due to the presence of minute inclusions. This 

 character is exactly similar to that noted by Sir T. Holland in the 

 felspars of some of the other rocks described in the same paper, 

 notably in the olivine norites from Singapuram, Salem district (Reg. 

 No. 9/398, p. 24), and Vitlapuram, South Arcot (Reg. No. 9/811, 

 p. 25) ; but whereas in the two latter rocks the colour is more 

 pronounced in the centre of the crystals, fading away towards their 

 edges, in the Nam Hsan rock it is concentrated along the edges 

 of the crystals. The peculiar colour of the olivine is more pro- 

 nounced in these rocks from Salem and South Arcot than in that 

 from S. Rewa. One important point of difference between the 

 Nam Hsan rock and those from S. India may be noted, namely, 

 that in the former the felspar has crystallised before the augite, 

 which surrounds it ophitically, whereas in the latter the felspar has 

 been the last mineral to consolidate. Magnetite in fairly large 

 quantities, and a little biotite, are the only accessory minerals that 

 I have been able to detect in the Nam Hsan rock. 



