REPORT OF THE CHIEF ASTRONOMER 233 



SESSIONAL PAPER No. 25a 



The quantitative mineral composition by weight percentages was deter- 

 mined (orthoclase not separately estimated but included in the andesine) 

 thus : — 



Hornblende 494 



Biotite 22-0 



Andesine 16-5 



Quartz 11-7 



Apatite -3 



Magnetite.. -1 



100-0 



The abundant biotite and quartz go far to explain the differences between 

 the chemical analysis here and that of the normal gabbro. It also appears- 

 from the analysis that the hornblende is here unusually aluminous. Chemically 

 considered this intermediate rock has its nearest relatives among the diorites; 

 yet the low feldspar content forbids our placing this rock variety in that family. 

 Like both the gabbro and the granite it is an anomalous type. 



In the Norm classification the intermediate rock appears in the as yet 

 unnamed sodipotassic subrang of bandase, the docalcic rang of the dosalane 

 order, austrare, with the following norm: — 



Quartz 9-72 



Orthoclase 13-34 



Albite 12-05 



Anorthite 28*63 



Corundum 1-53 



Hypersthene.. 26-51 



Ilmenite 1-22 



Magnetite 4-18 



Apatite -62 



H 2 and C0 2 1-39 



99-19 



At the perpendicular distance of 200 feet from the lower contact of sill E,. 

 another specimen of the intermediate rock was collected. It gave the following- 

 weight percentages (mode) : — 



Hornblende 42-9 



Quartz 22-8 



Andesine 18-5 



Biotite 6-6 



Sodiferous orthoclase 5-5 



Titanite . .. .. 3*7 



1000 



ABNORMAL HORNBLENDE GABBRO. 



The whole of sill B, and the lower part of each of sills C, D, and E are- 

 all constituted of dark, heavy gabbro (Phase 6), which is either sensibly like 

 the usual gabbro of the thinner Purcell sills, or differs from it in unessential 



