Vol. 50.] BANDED STRUCTURE OF SOME TERTIARY GABBROS. 651 



of pyroxene : they are aggregates of minute prisms or fibres of 

 green hornblende, with which some chlorite is usually associated. 

 The prisms are not, as a rule, orientated in any definite direction, 

 but cross each other in one and the same pseudomorph. Such 

 pseudomorphs are not uncommon in the coarser varieties of gabbro, 

 and they will be referred to in the following description as con- 

 sisting of pilitic hornblende. The magnetite occurs in smaller 

 individuals than the other constituents and occasionally shows traces 

 of crystalline form. 



These rocks are remarkably uniform in composition (sp. gr. 3), 

 and their external resemblance to ordinary basalts has been already 

 referred to. Their typical granulitic structure, the absence of the 

 characteristic lath-shaped sections of felspar, and the frequent 

 occurrence of the diallagic modification of pyroxene are the features 

 which have led us to call them gabbros ; but it must be remembered 

 that they are sharply marked off from the rocks which remain to be 

 described by reason of the small size of the individual constituents. 



The Banded Gabbros. — These are coarse-grained rocks composed 

 of pyroxene, plagioclase, olivine, and magnetite. Hornblende in 

 three forms, chlorite, and epidote occur as secondary or accessory 

 constituents. 



The pyroxene is pale brown in colour, and, so far as our obser- 

 vations go, is in the condition of ordinary augite. The individuals 

 are often elongated in the direction of the vertical axis, and cross- 

 sections occasionally show an approach to the common eight-sided 

 form ; but the angles are always rounded. A tendency to ophitic 

 structure is not uncommon. Twinning of the ordinary type may 

 sometimes be observed. Grains of magnetite and pseudomorphs of 

 pilitic hornblende, similar to those already referred to in describing 

 the granulitic gabbros, occur as inclusions. These pseudomorphs 

 may possibly represent olivine. The pyroxene has not unfrequently 

 been partially replaced by uralitic hornblende, and in one case this 

 change was seen to have taken place along cracks which could 

 be followed across the slide for a considerable distance. Where 

 these cracks traversed the augite, the pale brown substance of that 

 mineral was replaced for a short distance on either side by green 

 uralitic hornblende. 



Felspar occurs as grains, as irregular ophitic patches, and also in 

 forms which give broad rectangular sections; but these different 

 modes of occurrence are not as a rule found in one and the same 

 specimen. Twinning on the albite, pericline, and Carlsbad plans may 

 be observed. The perfection of crystalline form varies in different 

 rock-specimens. Where the felspar is most abundant (PI. XXVIII. 

 fig. 1), there the idiomorphism is most pronounced, and where it is 

 least abundant (PI. XXVIII. fig. 5) it is moulded on the other 

 constituents and shows no trace of crystalline form. The extinction- 

 angles indicate a variety closely allied to labradorite. In a few 

 specimens the felspar has been rendered turbid by alteration, but 

 as a rule it is quite fresh and water-clear. 



