180 PETROLOGICAL NOTES ON SOME OF THE ERRATICS 



The most abundant phenocrysts are plagioclase, twinned after both Carlsbad and 

 albite laws. There seem to be two sets of phenocrysts of plagioclase, the larger ones 

 are andesine Ab 5 An 3 on the outside and approach anorthite in the kernel and are 

 very strongly zoned. These have dimensions about l - 5 mm. by 0"75 mm., parallel 

 to (010), but are very thin, only about -15 mm. in the direction of the b axis. 



The other plagioclase phenocrysts are smaller and stouter than these, and are more 

 acid in character. 



The most striking mineral present is the amphibole, which is markedly composite. 

 The greater part of each crystal is brown in colour, but the outer zones, and particularly 

 the ends of sections from the prismatic zone, are dark green. In a section parallel to 

 (010) c : C = 14°. The pleochroism is : 



9 = brownish-yellow ; 

 t) = reddish-brown ; 

 C = dark clove-brown ; 

 and absorption a < tJ = C- 



These properties indicate that the mineral is barkevicite. 



The optical properties of the green portion are somewhat anomalous, and perfect 

 extinction between crossed nicols does not occur. The extinction angle appears to be 

 about 5° greater for the green than for the brown. Many of the crystals are twinned 

 after the common law, and, in addition, some curious interpenetration twins occur also. 



Quartz is not abundant, the grains average about 0*25 mm. in diameter, and are 

 granophyrically intergrown with orthoclase round the borders. In addition to the 

 orthoclase just mentioned there are a few independent crystals, twinned after the 

 Carlsbad law, and perfectly clear and undecomposed. 



A few apatite needles are scattered through the rock. 



Flakes of green penninite occur, indicating that biotite was an original constituent 

 of the rock in all probability. 



441. S6LVSBERGITE 



Microscopic Character s.— Somewhat similar to No. 452 but less trachy tic 

 in structure. The base consists of a confused aggregate of rather decomposed, un- 

 twinned orthoclase, abundant greenish chloritic material (probably derived from 

 amphibole) and very little quartz. 



The phenocrysts are kaolinised felspars with zonal decomposition near periphery. 

 The refractive index of these is less than that of quartz but equal to or greater than 

 that of Canada balsam. Some of them show multiple twinning but no good sections 

 for determination were encountered. 



A few grains of quartz which are present seem to be foreign inclusions. One fragment 

 included in the slide consists of quartz, complicated saussuritised felspar, a very large 

 apatite crystal and green hornblende quite distinct from that of the rock itself. 



Hornblende is abundant in perfectly idiomorphic crystals very similar to that in 

 No. 452, but without the well-marked green borders there described. 



430. Sapphire-bearing Trachyte 



Microscopic Character s. — The rock is markedly trachytic in character, with 

 a few porphyritic crystals. 



The base consists mostly of lath- shaped felspars up to 1 -2 mm. by -04 mm. Almost 

 all are sharply twinned on the Carlsbad law, and one or two of the largest show hazy 

 patches of what may be very fine multiple twinning, but this is doubtful. One of the 



