﻿106 



MB. G. W. TYRRELL ON THE 



[vol. lxxii, 



titanaugite, olivine, nepheline, and ilmenite, with accessory horn- 

 blende, biotite, analcite, orthoclase, and apatite. The texture is 

 very fine-grained, and hence the rock might be termed nephe- 

 line-dolerite or cloleritic theralite. In the principal variety 

 the fabric shows innumerable small, prismoid grains of augite em- 

 bedded poikilitically in a ground-mass of felspar and nepheline, the 

 whole forming a base in which pseudoporphyritic olivine, augite, 

 and ilmenite is set. The hornblende-bearing variety has the same 

 fabric, only a new pseudoporphyritic element, hornblende, being 

 added. The light-grey veins, however, contain no granular augite, 

 and their abundant analcite gives them a fabric similar to that of 

 the teschenites. 



In the principal variety the felspar forms broad, anhedral plates, 

 and seems to have been the last constituent to crystallize, even the 

 nepheline being euhedral towards it. It is extremely zonal, and is 

 crowded with grains of augite. Its composition is consequently 

 difficult of determination, bat the evidence points to the usual 

 acid labradorite (A^Anj), with marginal transitions to oligoclase. 

 The plagioclase is liable to decomposition, especially in the central 

 portions of the crystals, and passes into an irregular felted mass of 

 scaly mica. 



The augite occurs in two forms : as innumerable prismoid 

 grains embedded in felspar and nepheline ; and as larger, pseudo- 

 porphyritic, euhedral crystals of a much darker tint than the 

 grains. The colour is a pale purple or lilac, generally darker 

 towards the margins of the crystals. The mineral is often strongly 

 pleochroic, from purplish to pale yellow. It alters, just as in the 

 teschenites, with the production of biotite. 



Olivine occurs in large pseudoporphyritic crystals, euhedral 

 originally, but now the angles are largely rounded off, and 

 therefore nearly spherical grains are not uncommon. It is very 

 •fresh with, at most, a slight peripheral serpentinous alteration. 

 All the fissures are much blackened by separated magnetite. 



Nepheline forms small subhedral to anhedral masses, and appears 

 to be idiomorphic towards the felspar. It is usually altered to a 

 turbid, streaky, micaceous aggregate, the scales of which are 

 arranged parallel to the crystallographic axis of the mineral. It 

 is occasionally quite fresh, and its identification can be made abso- 

 lutely certain by the usual tests. 



Ilmenite occurs in skeletal masses associated with biotite, but 

 not so abundantly as in the teschenites. Biotite also occurs as 

 small independent flakes embedded in felspar or nepheline, or as an 

 alteration -product of augite. 



Analcite occurs very sparingly in the principal variety of 

 theralite, but is much more abundant in the veins towards the 

 top of the stratum. Apatite is very abundant, enclosed in all 

 constituents. 



A red soda-hornblende, belonging to the barkevikite group, 

 becomes an important constituent in the lower part of the theralite 

 stratum. It usually forms extremely irregular plates, embayed by 



