36 Rocks of Noyang. 



u 



where the composition face is co P go with interpenetration 

 of twins, so that a double twin is produced in which the two 

 diagonally opposite are optically similar ; and (d) the Albite 

 law and the Pericline law combined. 



(2.) Felspars which are not well crystallised, but which 

 are certainly simple, and in some of which, so far as could 

 be made out from an examination of such minute objects, 

 obscuration occurred where the longer diameter was in 

 accordance with the plane of polarisation of the nicol. 



(3.) Very rarely small imperfect crystals of a chloritic 

 mineral. Its characters are precisely those which I shall 

 describe when speaking of the porphyritic minerals of " first 

 consolidation" in this rock. These chlorite flakes are pro- 

 bably the alteration products of such detached flakes of 

 mica as I have observed to exist in the quartz of the quartz- 

 mica-diorite. 



(4.) A few minute, colourless prisms, of a tetragonal habit,, 

 which polarise brightly, and become obscured when the 

 plane of the nicol is parallel or. perpendicular to their 

 prismatic sides. In one instance the measurement of the 

 angle go P A P gave me 132°. These crystals are certainly 

 zircon. 



The porphyritic minerals of the first- consolidation are as 

 follow: — 



Felspars. These are compound crystals, usually tivinned 

 according to the combined Albite and Carlsbad laws. The 

 edges and corners are mostly rounded off or broken. They 

 also form groups of several crystals, adjoining each other 

 with their fractured ends. The angles of obscuration I 

 found to be in the zone OP — oo P oc 5° 45' to 16° 15'. These 

 porphyritic felspars are somewhat altered, being not only 

 kaolinised to some extent, but also full of minute flakes 

 of a pale green colour, a mineral probably of the chlorite 

 group. The optical and physical properties of these felspars 

 may point to oligoclase rather than to albite. 



Quartz occurs in more or less perfectly formed but often 

 corroded and fractured crystals, with hexahedral outlines, 

 and their character is precisely that so familiar to observers 

 in the quartz of the quartz-porphyries. 



Chloritic Minerals. Besides these there are a number of 

 crystals of a chloritic mineral of about the same size and 

 relative number as those of quartz. These are evidently the 

 alteration products in situ of a magnesia-iron-mica, but in 

 not one instance in the slices I prepared of this variety of 



