REPORT OF THE CHIEF ASTRONOMER 223 



SESSIONAL PAPER No. 25a 



expense of the other, which has been simultaneously decolourized. No trace 

 of any pyroxene or of pseudomorphs of pyroxene has been discovered in any 

 slide. Many of the sliced specimens are so fresh, as shown by the preservation 

 of the essential minerals as well as of biotite, that the pyroxene must be dis- 

 coverable if it had ever entered into the composition of the rock at the time 

 of crystallization from the magma. Another hypothesis, that some of the 

 fibrous hornblende has resulted from the speedy alteration of originally crystal- 

 lized pyroxene, through the influence of magmatic vapours which acted long 

 before the rock was exposed to ordinary weathering, cannot be excluded. So 

 far, however, the positive microscopic evidence declares in favour of the first 

 view. Similar cases of the derivation of fibrous amphibole from compact amphi- 

 bole through metasomatic changes are described by Zirkel.* 



The hornblende is, in the prismatic zone, idiomorphic against the feldspar; 

 it fails to show good terminal planes. The ends of the crystals characteristically 

 run out into narrow forked blades. The extinction on (010) averages about 

 13° 30'; that on (110), about 11°. In phases of the rock where quartz is an 

 abundant accessory, the amphibole is often highly poikilitic, the prisms being 

 charged with swarms of minute droplets of quarte. For the purpose of finding 

 the optical orientation the attempt was made to produce etch-figures on the 

 more likely looking specimens of the amphibole but, on account of the poikilitic 

 and blady character of the amphibole, the attempt was not successful. 



From the chemical and quantitative analyses of the type rock, a rough 

 calculation of the chemical composition of the hornblende gave the following 

 proportions : — 



Si0 2 49-8 



AUG, 5-2 



Fe 2 5-2 



FeO.' 12-1 



MnO -2 



MgO 15-3 



CaO 11-9 



99-7 



The estimate is crude but it shows that the amphibole is a common horn- 

 blende high in silica, iron, magnesia, and lime, but low in alumina. 



The feldspar is plagioclase, always well twinned on the albite law and 

 often on the Carlsbad law. Many individuals extinguish with angles referring 

 them to labradorite, Ab 3 An 4 ; some have the extinction angles appropriate to 

 basic bytownite; a very few others are zoned, with anorthite in the cores and 

 andesine in the outermost shell. The average composition of the plagioclase 

 in the normal rock is near that of the basic labradorite, Ab t An 2 . 



Magnetite, titanite, pyrrhotite, and apatite are all present but are strik- 

 ingly rare in most of the slides. Their forms and relations are those normal 



*F. Zirkel, Lehrbuch der Petrographie, Vol. 1, 1893, p. 325. 



