24 MIDDLKMiss : THE GEOLOGY OF JDAIi STATE. 



area. The rolling country is very irregular and considerably 

 wooded, but numerous footpaths and cattle-tracks may be found 

 traversing the area from Walren and Dijio to Talau. whereby it 

 may be visited and examined in spite of the confusing details of 

 the topography and the ever-present jungle. In this respect it 

 is much easier of access than the quartzite hills to the east which 

 are much wilder and steeper. 



The biotite-gneiss of this formation is in every respect a well- 

 foliated, medium-grained. black and white 

 .Mineral oompom- rock thoroughly gneissic to texture. It 



tion. . . 



never becomes of granitoid texture, nor are 

 the distinctive and varying lamiiuo of different mineral composi- 

 tion suggestive of a crushed and foliated granite, from which it 

 differs also in other important details of composition. These lam- 

 ina) are frequently wavy or contorted, as regards the foliation planes. 

 Specimen No. ^\ (12289, PI. 9, fig. I), from about 1 mile E.N.E. of Dijio, 

 in the stream-bed, may be taken as a typical sample of one variety 

 of the rock. Biotite and quartz are the principal, and most pro- 

 minent, minerals visible to the eye. Under the microscope the 

 same is seen to be true, the biotite appearing in great plates giving 

 pale greenish-yellow and dark' chestnut-brown pleochroism. much 

 the same as that in some specimens of the calo-gneiss. There is 

 also a quite perceptible amount of colourless mica in smaller aggre- 

 gates of plates. The quart/ is in large areas of interlocking grains, 

 but distributed among it is a fair sprinkling of. sometimes more 

 or less regularly bounded and isolated, short felspar grains. These 

 show broad polvsynthetic twin lamella 1 on the albite plan. In 

 sections giving approximately rectangular cleavages, and therefore 

 nearly at right angles to the base (001) and the composition plane 

 (010), the extinctions of the twin lamella; on each : ide of their trace 

 make angles of about 20° and 37° respectively, the average being 

 28°. A good cleavage across one of these sets of plates gives an 

 angle of about 97° or 96° with the trace of the lamella 1 and 

 another across the other set gives an angle of about 90° with that 

 set, the average being about 93°. This may be presumed to 

 represent the true angle, a, between the base and the second 

 pinacoid, (001) A (010). In sections showing no t win lamelhe, and 

 which may be presumed to be parallel to the plane of composition 

 b or (010). the extinction angle is from 2C° to 32° with the basal 

 cleavage. These data agree, in indicating a lime-soda felspar with 



