﻿98 



ME. G. W. TFKRELL ON THE 



[vol. lxxii, 



(2) Teschenite. (PL X, fig. 2.) 



The differences between the teschenites proper of the upper and 

 those of the lower margins respectively are so slight, that the rocks 

 may be described most conveniently in the same section. The 

 teschenites are rather variable, both in macroscopic appearance and 

 in thin section. Variation occurs in granularity and fabric, but the 

 mineral composition is approximately constant, or, at least, variations 

 in the minerals present or their relative abundance do not carry any 

 of the rocks outside the teschenite group. The dominant type, 

 perhaps, is one which has an effect of very coarse grain, due to the 

 abundance of large, irregular, black prisms or blades of augite 

 ranging up to an inch in length, embedded in an even-grained, 

 apparently-uniform ground-mass consisting of pinkish felspar and 

 analcite. It is noteworthy that the teschenites of the upper 

 margin carry a pink analcite, whereas at the lower margin the 

 analcite is white. The former, therefore, are mottled in black, 

 green (olivine, serpentinous and chloritic alteration-products), and 

 pink; the latter in black, green, and white. The dominant type 

 with columnar augites (Galston type) 1 is interbanded in the upper 

 teschenite with a much finer-grained rock devoid of the pseuclo- 

 porphyritic augite. Towards the junction with the underlying 

 theralite the pink analcite becomes increasingly abundant, and 

 forms the dominant macroscopic element in a thin irregular band 

 immediately overlying the theralite. In the lower band of 

 teschenite a fine-grained type occurs, carrying numerous, small, 

 acicular prisms of augite in a white ground-mass of felspar and 

 analcite, with much hornblende and biotite, and a little nepheline 

 (Cathcart type). 



In thin section the teschenites consist essentially of labradorite, 

 titanaugite, analcite, olivine, and iron-ores, named in order of 

 abundance. As accessories occur biotite, orthoclase, apatite, 

 nepheline, and barkevikitic amphibole. The last-named invariably 

 occurs as a peripheral alteration -product of the titanaugite. The 

 secondary products are serpentine, chlorite, and leucoxene. The 

 texture is medium- to coarse-grained, and the fabric an inter- 

 locking mesh of labradorite and titanaugite in sub-ophitic relations, 

 the large polygonal or irregular interspaces being filled with analcite. 



The plagioclase forms broad laths, thoroughly euhedral, and 

 highly zonal. It is a medium to acid labradorite (Ab,An.— AfyAiij).- 

 The crystals are frequently somewhat albitized, and, adjacent to 

 large analcite areas, have been irregularly corroded and replaced 

 by analcite. The cleavage-cracks and the interior of a crystal are 

 occasionally occupied by serpentinous material, which has migrated 

 from adjacent decomposing olivine. 



The pyroxene is a feebly-coloured titanaugite, which, in the 

 dominant type, occurs as large, blade-like, or columnar crystals. 

 These, however, although elongated in one direction, are very 



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



