246 ME. H. H. AENOXD-BEMROSE ON A SILL AND [May 1 899, 



largest crystals attaining a length of 2 mm. The augite is 

 in grains. The olivine is altered to the Potluck pseudo- 

 morph, sections at right angles to the optic axis giving a 

 black cross and negative double refraction in convergent 

 light.^ Eods of magnetite are present. 



iVb. 888. — A specimen from the top of the quarry (sp. gr. 2-83) 

 is a typical ophitic olivine-dolerite, in which the felspars are 

 generally small. The pseudomorphs of olivine are serpen- 

 tinous or of the PeakPorest type.^ The magnetite sometimes 

 is penetrated by one or more felspars. (PL XX, fig. 1.) 



No. 889. — A few feet farther down (sp. gr. 2-80) the augite- 

 plates, which are sometimes twinned, become smaller, and 

 the felspars larger. Potluck pseudomorphs of olivine are 

 present. 



No. 887. — About 14 feet below the top of the quarry (sp. gr. 

 2*84). The augite is in small grains, or in nests or groups of 

 large interlocking grains, and penetrated by felspars only in 

 very few instances. The felspars sometimes attain a length of 

 2 mm., and predominate over the other minerals. The olivine 

 is altered to the Potluck pseudomorph. (PL XX, fig. 2.) 



No. 896. — 17 feet below the top (sp. gr. 2-88). The augite-grains 

 and felspars are smaller, and tbe olivine is altered to ser- 

 pentine. 



Nos. 897 Sf 900.— 20 feet below the top (sp. gr. 2-80). The augite 

 is in grains and small prisms, is Jess transparent, and has a 

 faint reddish-brown colour in ordinary light. The olivine is 

 altered to iron-oxide. 



No. 898.-29 feet below the top (sp. gr. 2-83). The rock is 

 similar to the preceding. 



No. 899. — 31 feet below the top, and probably about 10 feet 

 above the limestone. The rock is a fine-grained dolerite 

 (sp. gr. 2-86). The felspars are small, and the olivine is 

 altered to serpentine and iron-oxide. 



The fine-grained dolerite immediately above the limestone on the 

 opposite side of the valley. No. 34 (sp, gr. 2'86), is similar, but the 

 olivine, partly altered to serpentine, has often a fresh nucleus. 

 (PL XX, fig. 3.) 



Nearly every thin slice of the coarse and fine-grained types of 

 this dolerite contained small quantities of a faint blue or yellow 

 material, which is blue or brown in polarized light, with vermicular 

 stracture ; it probably represents altered patches of interstitial 

 matter. 



It is impossible to examine the upper part of the sill with the 

 same minuteness as the lower portion, which is so well exposed 

 in the quarry. The only evidence to be obtained is from the isolated 

 blocks which project above the soil, on the hill-slope. Hand- 

 specimens from these were compared with those just described. 

 The rock becomes less coarse in structure as its upper surface is 



1 Quart. Journ. Geol. Soc. voL 1 (1894) p. 613. ' Ibid. p. 614. 



