Bocks of Noyang. 45 



spherulitic aggregates very often touch each other, while 

 elsewhere they are so far apart as to admit the interposition 

 of crystals of plagioclase or micro-crystalline masses of 

 quartz and felspar. In some few instances imperfectly 

 formed crystals of felspar form the centre of the aggre- 

 gates; more rarely these latter are arranged in the manner 

 described as "structure pegmatoide." Quartz does not occur 

 in this rock porphyritically. 



No mica or amphibole is present, nor any alteration pro- 

 ducts which would indicate their former existence. 



(c.) Quartz- P or phy rites. 



These rocks are so intimately connected with the quartz- 

 mica-porphyrites that they might with propriety have fol- 

 lowed them in this description ; but as I have considered 

 these intrusive rocks in the order in which I believe them to 

 have been formed, the quartz-porphyrites in their most 

 typical examples find their place here. 



These rocks I have observed as dykes cutting across the 

 older igneous rocks. At a certain part of the course of the 

 stream which is known as the Mount Elizabeth branch, it 

 leaves the crystalline-granular quartz-mica-diorites of its 

 upper valley, and flows over successive masses of breccias, 

 of black vitreous-looking porphyritic rocks, of quartz-mica- 

 porphyrites, and of masses and dykes of milk-white, close- 

 grained quartz-porphyrites. These ]ast I believe to be the 

 youngest of the whole group, with the exception of the 

 black, vitreous rocks (felsophyrite). It seems to represent 

 the magma of these older rocks freed from the basic con- 

 stituents which in them go to form the mica, amphibole, and 

 magnetite. 



I now give the results of microscopic examination and 

 chemical analysis of these white quartz-porphyrites. The 

 samples were collected from two localities : — 



(1.) A slice prepared from this sample I found to be com- 

 posed of felspar and quartz. In places the former showed a 

 tendency to crystallise out in definite forms, showing that it 

 had a slight priority in order of formation ; but although 

 these imperfect crystals showed twinning, I could not obtain 

 any measurements of the angles formed by the planes of 

 vibration in the twin halves. In the ground-mass there was 

 one porphyritic crystal of plagioclase, but, unfortunately, 

 the greater part of it was lost in preparing the slice. A few 



