48 Rocks of Woywmg. 



This rock may be considered as essentially a compound of 

 felspar and quartz. The percentage composition can be 

 taken as follows in the unaltered rock, assuming the kaolin, 

 as before, to represent the alterations in the felspar, and 

 disregarding the hydrated iron ore : — 



Felspar 61-91 



EO minerals ... ... ... ... -47 



Quartz 37-62 



100-00 

 The molecular proportion of these minerals I find to be 

 as follows : — 



Felspar, 13343 ; Quartz, 88'88 ; RO Minerals, 1- 



It is to be noted that this rock not only consists essen- 

 tially of felspar and quartz, but that the felspar is almost 

 wholly albite, it being most probable that the K 2 is here 

 isomorphous with the Na 2 in a triclinic form. 



The following comparison of the constitution of the four 

 allied rocks may now be made, and the increasing acidity of 

 magma comes out clearly. The analysis No. 5 may be 

 thought to nearly represent the ground-mass of the por- 

 phyritic members of the series : — 



Felspar. Quartz. RO Minerals. 



Analysis No. 2 ... 1-30 ... -42 ... 1- 



No. 3 ... 8-61 ... 4-31 ... 1- 



„ No. 4 ... 12-5 ... 7-9 ... 1- 



No. 5 ... 133-43 ... 88-88 ... 1- 



(d.) Qwartz-Felsopliy rites. 



The locality where this rock occurs appears to have been 

 a focus of igneous activity. It is here that the great mass 

 of quartz-mica-porphyrite extends out westward with its 

 associated dykes, first filling a wide space in the crystalline- 

 granular rocks, and then separating them from the sediments. 

 At this place it is also that the masses and dykes of white 

 quartz-porphyrite have penetrated the older igneous rocks. 



The felsophyrite masses seem to me, so far as I could 

 make out in the absence of a detailed feature survey, to fill 

 in a central position in regard to the above-mentioned 

 formations, and to be flanked by masses of breccias, which 

 at first sight have the appearance of being made up of 

 angular fragments of compact porphyrites, but which on 



