218 J. J. H. TEALI-—PETROLOGICAL NOTES ON 
to the chemical properties in the manner laid down by Schuster 
(Min. Mitth. 1881, p. 117), then it indicates that the outer portion 
of the crystal approaches more closely to the composition of anorthite 
than that which forms the main mass. It is worthy of note that 
in this crystal, as in most of, if not all, the others which show the same 
character, although the outer angles are tolerably sharp, those of the 
internal portion (the nucleus, as it may be termed) are more or less 
rounded. In another section three zones may be recognized ; the 
outer and inner ones extinguish simultaneously, but the middle one 
extinguishes at an angle which shows a nearer approach to anor- 
thite. The outer edges of crystals zoned in this particular manner 
are frequently ragged owing to inlets of the ground-mass; these 
inlets, however, are limited to the outer zone and do not penetrate 
the central portion. It is clear therefore that the outer zone was 
added during one of the later stages in the process of rock-consoli- 
dation, and probably after some of the constituents of the ground- 
mass had separated out. 
Small Felspars.—These vary in character according as the section 
is taken from the main mass or from the margins of the dyke. In 
the latter case they consist for the most part of small microlites, 
whereas in the former they may be (a) fairly well developed crys- 
tals, not very long in proportion to their width, (6) imperfectly 
developed and skeleton crystals, (¢) long narrow acicular microlites. 
The more perfectly developed crystals are frequently ragged at their 
extremities, and sometimes run.out into the long narrow microlites, 
thus proving the felspathic nature of the latter. The sections of the 
more perfect crystals are often lath-shaped in form, and usually le 
between the following dimensions :— x -1 mm. x ‘35 mm. and*2 mm. 
x :dSmm. The small felspars are distinguished from those which 
give the rock its porphyritic character by the general absence of 
inclusions, and of zonal banding, as well as by their size. They 
occasionally show the repeated twinning characteristic of plagio- 
clase, but more frequently occur as binary twins and simple indi- 
viduals. The following numbers represent a series of measurements 
of the extinction-positions of adjacent lamellz, with reference to the 
twinning-line, those crystals being selected which gaye approxi- 
mately symmetrical extinctions: 21° and 34°, 22° and 35°, 26° and 
29°, 38° and 28°, 21° and 19°. Another series of observations in 
which the angle between the extinction-positions of adjacent 
lamelle alone was recorded, gave 27°, 19°, 6°, 58°, 36°, 46°, 22°, 
41°, 44°, 47°. It thus appears that 58° was the maximum observed, . 
The maximum for labradorite in the zone which gives symmetrical 
extinctions with reference to the twinning-line is 62° 30’, that for 
oligoclase 37°, for albite 31° 30’, and for anorthite over 74° 42’, 
The measurements therefore appear to indicate the presence of 
labradorite, but they can of course tell us nothing as to the presence 
lead him to the conclusion that they may be accounted for, in certain cases, on 
the assumption that the crystals are built up by the association of submicroscopic 
twin lamellz. 
