﻿part 2] 



PICRITE-TESCHENITE SILL OE LTJGAK. 



107 



the earlier constituents, and occasionally enclosing them. It is 

 pleochroic, with colour-extremes of pale straw-yellow and clear 

 red-brown. 



The quantitative mineral composition of the main varieties of 

 theralite is set out in Table III, cols, i & ii (p. 109). It will be 

 seen that the mass of the rock is decidedly mafic, much more so 

 than the rocks which have been described as theralite. In addition 

 to the mafic composition, it is characterized by a poikilitic fabric. 

 The light-grey veins, however, are much coarser in grain, are devoid 

 of granular augite and the poikilitic fabric, and show approximate 

 ecpiality between the felsic and the mafic constituents. Contain- 

 ing, as it does, some orthoclase and analcite, this variety closely 

 resembles the true theralites, which are, in general, mafelsic in 

 composition. A mineralogical feature that deserves special mention 

 is the beautiful lilac colour of the augite. 



(4) Lugarite. (PL XI, fig. 1.). . 



This rock is found intercalated near the transition between the 

 theralite and the underlying picrite. It has a maximum thickness 

 of 4 feet, and is intimately welded to both the contiguous rocks. 

 It also occurs as irregular, anastomosing veins ramifying through 

 the picrite, and varying in thickness from 1 to 5 inches. 



In hand-specimens it presents a striking and beautiful appear- 

 ance ; so much so that it is a matter for comment that the rock has 

 apparently never been noticed before. Boulders of it occur in the 

 bed. of the Lugar for a mile or two below the outcrop. It is 

 phanerocrystalline and apparently coarse-grained, consisting of an 

 abundant, continuous, greyish-green ground-mass (analcite, nephe- 

 line, and alteration -products) crowded with shining black prisms of 

 barkevikite ranging up to 3 inches in length. The rock of the 

 main exposure also shows equally abundant, more or less equi- 

 dimensional, black crystals of titanaugite. Occasionally, whitish 

 rectangular felspars may be recognized. Weathered blocks are still 

 more striking in appearance, as the ground-mass becomes white, 

 contrasting effectively with the black prisms embedded in it. 



Microscopically, lugarite is a very beautiful rock, owing to the * 

 brilliant colours of its mafic constituents and their perfect 

 euhedrism. The rock consists essentially of an abundant cloudy 

 greyish base, partly isotropic, and partly cryptocrystalline because 

 of an extremely-fine dusty alteration-product, clearing occasionally 

 to areas of identifiable analcite. This base is crowded with per- 

 fectly euhedral crystals of deep purple titanaugite, red barkevikite 

 ranging up to 3 inches in length, ragged masses of ilmenite 

 passing over to leucoxene, corroded felspars, and innumerable 

 prisms and needles of apatite. 



Titanaugite forms well-shaped crystals ranging up to a quarter 

 of an inch in diameter. Its pleochroism is very intense, the 

 scheme being as follows : — 



X clear pale brownish yellow. 



Y deep maroon or reddish purple. 



Z brownish violet. 



