71 



centage of iron in the analysis. Decomposition has, to a 

 slight extent, gone on, resulting in the formation of bastite. 



The plagioclase, which is not plentiful as a phenocryst 

 in this particular rock, has, in most instances, a well-defined 

 idiomorphic outline. The phenocrysts almost always contain 

 inclusions of apatite and dark-brown glassy base. In polar- 

 ized light twinnings after the Carlsbad and Albite law are 

 found to be common, Avhilst twinning after the pericline 

 law is not so frequently met with. In most cases they are 

 beautifully zoned (plate xx., fig. 2). Symmetrical extinctions 

 up to 40° were obtained, therefore the plagioclase is probably 

 a labradorite. 



The base consists essentially of tabulae of plagioclase fel- 

 spar, twinned usually after the Carlsbad law, and giving a 

 maximum extinction of 37°, which means that it is probably 

 a labradorite a little more acid than the phenocryst. They 

 possess, for the most part, an idiomorphic outline, and only 

 slightly decomposed. A few granules of augite are also pre- 

 sent, together with a few scattered grains of magnetite which 

 are, no doubt, titaniferous, because of the presence of leu- 

 coxine. The interstitial material of the base is particularly 

 interesting, in that it has, for the most part, become devitri- 

 fied to a granular or fibrous mass possessing many micro- 

 scopic grains of magnetite and a few needles of apatite. 



The slides examined, representing rocks from different 

 points of the occurrence, were more or less similar. Conse- 

 quent! 3 T there seems little doubt, if any, that the whole occur- 

 rence is contemporaneous. However, a few differences of 

 minor importance occur. In rocks Nos. 6 and 10 decomposition 

 has gone on to a great extent. Haematite is present in red 

 scales, and also staining the augite yellowish-brown. Some 

 of the augite has altered to serpentine and the enstatite to 

 bastite. Again, in rock No. 11 the felspar phenocrysts are 

 present in a greater proportion than the enstatite, whilst in 

 rocks Nos. 7, 8, and 9 the reverse is the case. 



Order of Crystallization. 



Magnetite ... ... 



Plagioclase (phenocryst) — — — 



Enstatite ... ... 



Augite ... ... ... 



Plagioclase (microlite) ... 



Glass ... ... ... — 



Chemical Analysis. 

 The rock chosen for analysis was selected from a point 

 about half a mile inland from the Bluff. Here the rock 



