118 ME. C. E. TILLEY OX THE GEAXITE-GXEISSES [vol. Ixxvii, 



magnetite has usually formed an inclusion in the primary augite. 

 A typical association is a magnetite-grain acting as a nucleus to a 

 perij)heral zone of red-bro\yn strongly-pleochroic biotite, and this 

 p)asses out into an amphibole-zone. 



Another rock (Xo. 41). which appears to be related to that just 

 ■described, shows a great deyelopment of hornblende, and the 

 blastophitic texture is Avanting. The constituents are hornblende, 

 plagioclase, pyroxene, and a few grains of iron-ore. 



The texture is typically granoblastic ; but there are a few lath- 

 like felspars with irregular borders, which may represent the last 

 surviyal of an ophitic structure. This is further suggested by the 

 fact that some of these carry minute o-ranular inclusions. The 

 centres of these irregular laths are occupied by strings of horn- 

 blende parallel to their length. The amount of plagioclase is 

 considerably less than in Xo. 15, and there is a noticeable paucity 

 of iron-ores. 



The granulitic pyroxene appears to be mainly an enstatite, and 

 types of aggregates similar to those seen in Xo. 45 are obseryed. 



The central sti'ings included in the lath-shaped felsjDars, which 

 are pyroxene in Xo. 15 and hornblende in Xo. 11, are not unlike 

 the metamorphosed equivalents of chlorite-strings, produced in the 

 weathering of felspars of igneous rocks. 



Xo. 85. — The same type of metamorphosed dolerite occurs in 

 bands in the gneisses of Boston Island. These are well represented 

 on the south-western shore. Xo. 85 in hand-specimens closely 

 resembles Xo. 15. Under the microscope, however, there are 

 important additional features which emphasize the high-grade 

 metamorphism that these rocks have undergone. The constituents 

 are augite, plagioclase, hypersthene, garnet, magnetite, biotite, and 

 a small amount of amphibole. 



The primary augite is characterized by its diallagic (100) lami- 

 nation and the deyelopment of ' schiller ' inclusions. The nature 

 of these is here more eyident, and they consist of magnetite or 

 titanomagnetite granules. The crystals of augite may again show 

 a rude radiate arrangement. 



The plagioclase is of the same composition as in Xo. 15 — a 

 labradorite with symmetrical extinction of 33°. This mineral was 

 separated from the remaining constituents by means of bromoform 

 solution. Under a high power it was found to be crowded with a 

 minute black dnst rendering it quite dark in colour. The refrac- 

 tive index exceeds 1'55, but is less than 1"57. In a liquid of 

 refractive index of 1*56, grains are present with an index both 

 above and below this value. The specific gravity exceeds 2"69, but 

 is less than 2*72. These properties, together Avith those ascertained 

 in thin slices, indicate labradorite. The range, however, iii A^iew of 

 the pronounced zoning, is from a basic andesine to a labradorite- 

 bytownite. It shows twinning after the Carlsbad, pericline, and 



