40 Rocks of Noyang. 



Estimating the kaolin as representing alterations in the 

 porphyritic felspars, and assuming these to have been of the 

 constitution, Alb. 3, An. 1, we have "88 molecules of oligo- 

 clase to be taken into consideration in deciding upon the 

 probable condition of the unaltered rock ; and similarly the 

 chlorite may be calculated as magnesia-mica. The result 

 of such a calculation gives the following as the probable 

 percentage constitution : — 



Felspar 63 36 



EO minerals 8-18 



Quartz 23-48 



100-00 



And these constituents are in the following molecular pro- 

 portion to each other : — 



Felspar, 8-61 : Quartz, 4-31 ; EO Minerals, 1. 



Another example of a rock of this group I found as a 

 dyke crossing Navigation Creek below the junction with it 

 of the Mount Elizabeth branch. In its least weathered 

 portions it is hard, compact, of a pale slate colour, and of a 

 flinty appearance. With the lens there are visible here and 

 there minute cleavage planes of felspar and glassy-looking 

 granules of quartz. 



Under the microscope I found it to have a micro-crystal- 

 line-granular ground-mass of quartz and felspar, in which 

 were a few porphyritic crystals, and here and there 

 slight traces of yellow basis. In this ground-mass there are 

 innumerable bladed microliths of some chloritic mineral. 

 These lie at all angles in the rock, are bent, twisted, and 

 often ragged at the ends, and are of a pale green colour. The 

 constituents of this ground-mass show in places traces of 

 radial structure. The porphyritic crystals are rare and of 

 the subjoined kinds : — 



(1.) A single large porphyritic crystal of triclinic felspar, 

 in which the two compound halves are composed of plates 

 which are not continuous throughout. The optical angle of 

 obscuration on each side of the composition face of the 

 Carlsbad twin I found to be nearly 17°. 



Three small compound twins are probably of the second 

 order of consolidation. 



(2.) The porphyritic-quartz crystals are also very rare, 

 there being only two, and these are both much rounded 

 and eroded at the edges. 



