Rocks of JS T oyang. 43 



This rock may be assumed to have had the following com- 

 position, if we assume that the alumina and the magnesia in 

 the chlorite represent the mica : — 



Felspar 61*50 



RO minerals ... ... ... 3*10 



Quartz ... ... ... ... 35*40 



100-00 



And on this basis the molecular proportion would have 

 been — 



Felspar, 12-5 ; Quartz, 7*9 ; RO Minerals, 1. 



Quartz- Granophy rites. The rocks which I have selected 

 as illustrations of this subdivision of the group are the 

 following : — 



(1.) Contact of the quartz-porphyrite mass with the sedi- 

 ments at the Haunted Stream. 



In the slices of this rock which I prepared I found but 

 very little of a ground-mass such as that which I have 

 described in the quartz-mica-porphyrites, but that which 

 there is precisely resembles it in being a minutely fine- 

 grained compound of felspar and quartz. The far greater 

 part of that which in kindred rocks would go to form this 

 ground-mass is here aggregated into radial spherulitic masses 

 of felspar and quartz, which either form the whole or sur- 

 round some central object, or are disposed in irregular groups 

 round it. These central objects are in some cases quartz, in 

 others felspar crystals, or even chlorite, which, however, 

 cannot be regarded as an originally formed constituent. The 

 structure of these spherulites is not usually regular, but may 

 be described as being built up of several groups of radial 

 crystals. In some few the optical accordance of the various 

 parts is such that a more or less perfect black cross is 

 observable by polarised light in the spherulite as a whole, 

 but in most cases this is not the case, and there are indepen- 

 dent portions of several discordant crosses in the whole. 

 The quartz-crystals which form the centres of these spheru- 

 litic masses are precisely such as are found in the quartz- 

 porphyrites. The felspars are all plagioclase, and are often 

 fractured. In some cases I have observed two or three 

 felspar fragments forming the nucleus of a spherulite. One 

 very large instance I observed to be built up of four concen- 

 tric portions. The centre was a large somewhat rounded 

 crystal of quartz, having several of the characteristic 



e2 



