370 Transactions. — Geology. 



estimated as pyrophosphate. The variation in the results ob- 

 tained in this case has been explained. 



The alkalies were determined by the Lawrence -Smith 

 method, and. were determined separately, by titration with 

 silver-nitrate in the first two cases, and by precipitation with 

 platinic chloride in the last two. 



The presence of manganese and phosphorus was also tested 

 for, but with no result. The absence of the last would show 

 that apatite was absent. 



For reasons given above, it will be seen that the last 

 analysis is the most trustworthy. 



The porphyritic felspar was analysed separately, but I will 

 give the results when treating of it microscopically. 



B. Microscopical Examination. 



Sections of the rock were made by grinding dowm thin 

 fragments till they were semi-transparent, care being taken to 

 get specimens from rock which was weathered as little as 

 possible. These were examined under the microscope by 

 means of polarised light. The rock then appeared to consist 

 of a semi-crystalline ground-mass in which crystals of plagio- 

 clase, augite, and olivine were porphyritically distributed. 

 All through the rock were traces of weathering, as limonite 

 showed nearly everywhere in the ground-mass. 



I. Ground-mass. 



The ground-mass was semi-crystalline, but the amount of 

 interstitial glass was comparatively small, nearly the whole of 

 the space between the felspar and olivine being taken up with 

 an interlacing network of felspar microliths. These were pro- 

 bably oligoclase, as in almost every case the direction of the 

 extinction was nearly in a line with the length of the micro- 

 lith ; but, on attempting to determine what kind of felspar it 

 was, by means of the extinction of twin lamella?, results were 

 obtained not at all in agreement with this. In some cases 

 the angle of extinction was so high as to make it anorthite. 

 Besides, in many cases, the microliths exhibited undulose 

 extinction, and the determination of the true angle was 

 impossible. Since the occurrence of a more basic felspar 

 than the porphyritic felspar (I shall afterwards show that 

 this is labradorite) is unusual, I conclude that the majority 

 of the felspar is oligoclase, as determined by the first 

 method. 



In some cases the amount of glass was so very small that 

 it hardly appeared in the section at all, but in other cases it 

 occupied a, comparatively speaking, large space. It was full of 

 crystallites, which had no effect on the polarised light. The 

 glass, besides, was coloured brown, owing to the presence of 



