264 ME. w. G. SHAK"N"ON ON A [vol. Ixxviii, 



are cognate xenoliths obtained from the picrite, and have been 

 partly absorbed and carried forward by the dolerite intmsion. 



(c) Vein of salic facies : ' bostonite.' — A conspicuous feature is 

 the white vein dipping 35° south-south-eastwards, in the same 

 direction as the jointing of the dolerite — its weathered surface is 

 of a light buff colour. In the upper right-hand portion a much 

 thinner band is just visible ; this has weathej-ed to a darker colour. 

 The veins are strongly jointed perpendicularly to their surface, 

 giving a partly columnar effect. The fresh rock is bluish grey and 

 felsitic in appearance. There are no noticeable contact-effects,. 

 and the vein is sharply marked off from the surrounding rock ; but 

 the latter is so decomposed for over a foot below the surface that 

 n3 sections could be cut to determine the exact contact. That the 

 vein is later than the dolerite is inferred from the columnar 

 jointing and the sharpness of the contact. Both veins dip beneath 

 the quarry-floor, and cannot be proved to cut the picrite. 



{d) Fine-grained modification of the dolerite. — In the south- 

 western portion of the quarry the wall is low, and a portion about 

 a foot below the top is noticeably less weathered, of greener colour 

 and finer grain. This merges imperceptiblj'' into the ordinary 

 dolerite. AYithin a few feet, on apparently the same horizon, tlie 

 dolerite is coarse-grained. This fine-grained rock will be defined 

 as the 'mugearite ' modification. 



(e) The dolerite which forms the bulk of the intrusion calls for 

 no detailed description, its chief characteristic being the facility 

 with which it weathers to a rusty-brown ' wacke.' 



(f) Lower Quarry. — The rock here is uniform and more 

 compact than the dolerite from the upper quarry. It is of a 

 predominant green colour, much veined by calcite. So far as I 

 have examined it, the picrite or salic veins do not occur, the rock 

 being dolerite throughout. The slates at the contact are altered 

 to a depth of 2 or 3 inches only, as in many dyke occurrences. 



IV, Peteogeapht. 



(a) The picrites. — In hand-specimens these are dark-green 

 rocks, with black patches of olivine and augite showing slight 

 schillerization. In thin section the most notable constituent is 

 olivine in subhedral crystals and smaller grains, the smaller olivines 

 being partly enclosed in poikilitic augite-plates. 



The olivine is optically ^^ositive, with an angle in air of about 70"", 

 indicating that it belongs to the forsterite end of the series. Its 

 alteration -products are characteristic of the olivines of plutonic 

 rocks : at first a heav}' border of magnetite appears ; this gradually 

 extends into the interior, forming magnetite dendroids, and the 

 crystal becomes completely veined by magnetite ; later this is re- 

 sorbed in the formation of serpentine and calcite. The augite is 

 purplish, and is probably a titan-augite, as in the augite of the 

 essexites ; it gives extinction-angles wp to 35^ or 40°. The augite 



