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PICRITE-TESCHENITE SILL OF LTJGAK. 



99 



irregular in shape, owing to their indentation by the terminations 

 of felspar-laths. In some of the rocks, notably the finer-grained 

 varieties, the titanaugite is perfectly euhedral, and is moulded by 

 felspar. The colour is a pale purplish brown, and is very variable 

 and patchy. The most common appearance is of a darker tint 

 towards the margins, but bands alternatively of lighter and darker 

 colour may occur. This zoning coincides with a similar zoning 

 observable between crossed nicols, and frequently also with an hour- 

 glass structure. It is, therefore, connected with slight chemical 

 and optical variations in the crystals. There is occasionally a 

 distinct pleochroism from purplish brown to a pale sepia. A 

 notable feature is that practically all the crystals are hollow. In 

 the prismatic sections elongated cavities occur along the centre- 

 lines, and are filled with calcite, serpentinous alteration-products, 

 and occasionally even analcite. The cavities are more or less equi- 

 dimensional in the basal sections. They are frequently lined with 

 a highly pleochroic biotite, which extends in ragged indefinite 

 patches throughout the interior of the crystal, and is evidently an 

 alteration-product. The latter frequently has an astonishing 

 pleochroism in shades of brilliant blue, red, and peach-bloom tint, 

 but it is sometimes scarcely more than a discoloration of the augite, 

 so indefinite are its boundaries. The more individualized mica has 

 a more normal pleochroism, but its darker shades have a peculiar 

 6 beetroot ' tint. Serpentinous material also occurs within the 

 augite, but its relations show that it has migrated from adjacent 

 olivine, entering the crystal through the cleavage and other cracks. 

 The augite often thus presents a honeycombed appearance in the 

 interior of the crystals, the exterior being almost invariably sound, 

 and free from inclusions and honeycombing. 



A red hornblende belonging to barkevikite occurs as an altera- 

 tion-product upon the margins of the augite-crystals, especially 

 in the fine-grained rocks of the lower contact, where the augite 

 is euhedral. The boundary between the two minerals is always 

 exceedingly indefinite. A rock from the lower band of teschenite 

 above Bellow Bridge is so rich in hornblende that it deserves 

 the designation hornblende-teschenite. This rock is fine- 

 grained and contains nepheline, thereby approximating to the 

 Cathcart type. 1 



A thin band of aegirine-augite or segirine frequently occurs on 

 the margin of an augite-crystal, especially where it is adjacent to 

 an area of analcite. Small crystals of segirine are occasionally 

 enclosed in the analcite. 



Analcite fills up large and small polygonal spaces between the 

 felspars and augite ; but, where corrosion and analcitization of the 

 felspar has occurred, irregular or rounded spaces are formed. It is 

 mostly fresh, showing the cubic cleavage, and occasionally some 

 anomalous birefringence. The commonest alteration is to an ex- 

 tremely-fine, irresolvable, brown dust ; but, with a further degree 



1 G. W. Tyrrell, Geol. Mag. dec 5, vol. ix (1912) p. 74. 



