﻿108 



ME. G. W. TYRRELL 0~S THE 



[vol. lxxii, 



The hour-glass and kindred zonal structures are prominent, and 

 the exterior zone is always the more deeply coloured and pleo- 

 chroic. A green coloration frequently appears on the extreme 

 margin. Occasionally, an augite encloses the termination of a 

 felspar-crystal ; and in rare cases it is interdigitated with barke- 

 vikite, the junction between the two minerals being indefinite, but 

 the exterior margins euhedral. A few crystals are honeycombed 

 by irregular cavities filled with colourless isotropic material, pre- 

 sumably analcite. The titanaugite of this rock is identical in its 

 optical characters with that of the ijolite-dolerite (nepheline- 

 dolerite) of the Lobauer Berg, Saxony. 



The barkevikite forms perfect crystals, and is of a deep reddish- 

 brown colour. The pleochroism is intense, with the following 

 scheme : — ■ 



X pale yellow. 



Y deep chestnut-red. 



Z deep brown-red with a tinge of violet. 



The maximum extinction-angle in prismatic sections is about 

 11°. The simple twin parallel to 010 is prominent. The mineral 

 is, therefore, referable to the barkevikite group. 1 It occasionally 

 moulds the pyroxene, and is then evidently an alteration-product ; 

 but the great majority of the crystals are entirely independent of 

 pyroxene. On the other hand, the smaller crystals are frequently 

 enclosed in the felspar. 



The felspar, where uncorroded, forms well-shaped rectangular 

 laths, and is an extremely zonal plagioclase. Where the extinc- 

 tions can be measured they indicate a labradorite of composition 

 Al^An.,. On the margins, however, nearby straight extinctions 

 are obtained, indicating a transition to oligoclase. The felspar is 

 nearly always corroded, and all stages in its replacement by dusty 

 analcite and isotropic alteration-products can be followed. The 

 replacement begins along the cleavage and other cracks, and 

 advances until large irregular areas in the interior of the crystal 

 are replaced ; while, at the same time, the alteration proceeds from 

 the exterior of the crystal in such a way that the felspar remnants 

 as»e often crescentic in shape, and appear as if large pieces had 

 been scooped or gouged from their sides. The final stage is a 

 complete replacement of the ciwstal ; or, at most, small crescentic 

 fragments of felspar are left. Frequently, however, the ciystal 

 form is preserved, and is differentiated from the rest of the ground- 

 mass by a slightly paler tint. 



Olivine occurs very sparsely as rounded inclusions in the horn- 

 blende or augite ; and biotite in small flakes, or as an alteration- 

 product of augite. Both minerals may be completely lacking in a 

 slide. 



Ilmenite occurs in rather well-shaped hexagonal crystals, and is 

 in process of alteration to leucoxene, giving rise to 'a marked 

 striation (in black on grey) of three sets of lines in directions 

 intersecting at 120°. It appears to have crystallized in this rock 



before the hornblende or the pyroxene. 



1 A. Scott, ' Barkevikite from Lu£ar ' Min. Mag-/ vol. xvii (1914) pp. 138-42. 



