3-2 MCMAHON : PETROLOGICAL NOTES 0:' SOME PERIDOTITES, ETC., 



The above specimens are all samples of identically the same rock 

 and it will be convenient to describe them together. 



The matrix appears to the unaided eye to be compact in structure 

 and varies from a dark slaty to a dark greenish-grey. 



The porphyritic felspars are in thin tabular crystals, the face b 

 (oio) forming the platy surface. They sometimes attain a length of 

 4 centimetres and their average width is about 2, and their thickness 

 from o'2 to o'4 centimetres. Owing to traction, or pressure, whilst 

 the rock was in a plastic condition, the b (010) faces are generally in 

 the same plane, so that when the fractured surface of the hand-speci- 

 men coincides with that plane, platy crystals only are seen. On the 

 other hand, when the fractured surface of the specimen is at right 

 angles to that plane, only slender, lath-shaped crystals are visible. 

 Both these features are well seen in the hand-specimen 8—281. 



The porphyritic felspars, judging from the extinctions measured 

 from the twinning plane in suitable cases, and other features, belong 

 mainlv to the labradorite species, though a little andesine appears to 

 be also present. The labradorite belongs to the most acid variety, 

 the highest extinction angle obtained not exceeding 26 . 



The orientation of the large felspars is generally speaking 

 approximately parallel, but here and there they locally radiate at 

 various angles up to 90 from this general direction, indicating 

 the existence of local variation in the effects of traction on the flow 

 of the viscid uncooled mass prior to consolidation. 



The microscopical examination of thin slices shows that many of 

 the porphyritic felspars possess zonal structure. They have some- 

 times been cracked and shattered internally and contain marginal 

 inclusions of the magma, which also penetrated them in the form of 

 tongues. The cracks are sometimes filled with chlorite and some- 

 times with a structureless isotropic substance, which is probably 

 allied to zoisite. Portions of the felspars, in some cases, are fairly 

 fresh. Other crystals are much corroded and some have almost 

 become pseudomorphs of chlorite and zoisite. 



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