SOME NORTH-OF-ENGLAND DYKES, 2NF 
examination ; but after many failures I have succeeded in preparing 
one reliable slide. The section is approximately at right angles to 
the twinning-plane and parallel to the edge o P| «Pa. Two 
separate determinations of the extinction-angles of adjacent lamella, 
with reference to the trace of the twinning-plane, gave (a) 5° 10’ 
and 7° 20’, (6) 5° 35’ and 7° 10’. This shows that the section is not 
exactly perpendicular to the twinning-plane, because if it were, the 
extinctions would be symmetrical; nevertheless it may be regarded 
as sufficiently near for the purpose of comparison with the table 
given by Messrs. Lévy and Fouqué (‘ Minéralogie Micrographique,’ 
p- 230). We find on referring to that table that in labradorite the 
extinction-positions of adjacent lamella in the plane passing through 
ciemedee! 0 P| "oc Px, and normal to the twinning-plane, lies 
between 10° and 14° 30’, and differs markedly from the corre- 
sponding angle in the other well-defined species of felspar. In the 
case in question it may be taken as about 12° or 13°, and we are 
therefore led to the conclusion that the felspar is labradorite. It is 
perhaps hardly safe to infer that all the large felspars belong to the 
same species; indeed the results obtained by observing the angle 
between the extinction-positions of adjacent lamelle in the felspars 
exposed in the rock-sections seem to indicate the presence of a 
felspar more nearly allied to anorthite. The large felspars contain 
inlets and inclusions of the ground-mass, and frequently show the 
most exquisite zonal banding (see Pl. XII. fig. 3). In one typical case 
which is now before me this banding is due to brownish, more or less 
spherical or elliptical, granules, which vary in size from the smallest 
specks visible with a magnifiying power of 270 diameters up to those 
which measure °02 mm. x ‘01 mm. The larger granules are 
markedly elliptical, and their longer axes are set parallel to the 
longer edge of the crystal section. They keep this parallelism 
throughout the zone,a fact which shows that the elliptical form 
was determined by the unequal rate of growth of the crystal in 
different directions. There are also larger irregular inclusions 
which do not lie in definite zones. It is worthy of note that the 
distance between contiguous zones is greater in the direction of the 
length than in that of the breadth of the crystal section ; another 
fact indicative of the unequal rate of growth of the crystal. Some- 
times the large felspars are completely honey-combed by irregular 
ramifiying inclusions; at other times the centre only contains such 
inclusions. Not unfrequently they are surrounded by a narrow 
outer zone, which extinguishes at a slightly different angle from the 
rest of the section. Thus, in one case in which the extinction-angles 
of the main mass of the crystal were 30° 35’ and 33° 20' on either 
side of the twinning-line, those of the outer zone were some 3° or 4° 
greater on either side. If we assume* that this difference is due to 
difference in composition, and that the optical properties are related 
* In ‘Comptes Rendus’ for 23 Jan., 1882, M. Lévy points out that the 
variations in the angles of extinction in successive zones need not necessarily 
indicate variations in chemical composition. His mathematical investigations 
