448 Dr. A. Holmes and Dr. S. Smith— 
¢ 
felspar-phenocrysts or enclosures (as they probably are} have had 
a similar origin and history. Their composition, which is abnormal, 
and the resorption of their outer parts, show that the crystals were 
not in equilibrium with the material by which they are now 
surrounded. They must, therefore, have sunk down or floated up, 
and both specific gravity evidence and the probabilities of the case 
point to the latter process as the more probable alternative. 
Pyroxenes——Both orthorhombic and monoclinic pyroxenes are 
present. Hach is of a pale grey-green tint, but there is little difficulty 
in distinguishing the two species. The orthorhombic pyroxene is 
only slightly pleochroic, and its composition approaches enstatite 
rather than hypersthene. It builds stout prismatic crystals of 
hypidiomorphic outlines, and sometimes occurs as nucleus-crystals, 
around which aggregates of the monoclinic pyroxenes have grown. 
The latter are far more abundant than the enstatite and their small 
optic axial angle indicate that they are not diopside or augite but 
enstatite-augite. The existence of a series of mixed crystals between 
clino-enstatite and diopside is now well established, and natural 
aluminous representatives have been recognized as the characteristic 
pyroxene of many doleritic rocks. Already in 1884 Sir Jethro Teall, 
in his masterly study of the Whin Sill, showed by analysis that the 
monoclinic pyroxene contained approximately nineteen molecules 
of diopside-hedenbergite to ten of enstatite, with smaller amounts of 
alumina and ferric oxide. The same type of pyroxene is present in 
the Wackerfield dyke, and the similarity extends to the mode of 
occurrence, which is in small grains or larger irregular plates, and 
to the presence of a fine striation which characterizes certain parts of 
the larger plates. The colour is grey-green rather than pale brown, 
but brownish patches occur sporadically suggesting local concentra- 
tion of iron, and possibly titanium, oxides. 
The alteration-products of the two pyroxenes are worthy of 
attention. The monoclinic pyroxene is sometimes altered to fibrous 
green actinolite, especially around the borders. The orthorhombic 
pyroxene, however, tends to be more deeply altered, but only 
when it occurs as isolated crystals. The alteration-product is a 
brown fibrous mineral showing strong pleochroism, and giving 
invariably straight extinction. The birefringence is about 0-020, and 
it seems clear, therefore, that the mineral is anthophyllite. 
Black Ores——The opaque constituents consist entirely of 
titaniferous magnetite. From the crushed rock practically the 
whole of the mineral is abstracted by a bar magnet, and the small 
second crop obtained by the electro-magnet consists of grains 
which are largely felspar or augite. In thin section the titaniferous 
magnetite builds skeletal crystals and also occurs in irregular masses 
of very different sizes. It encloses both pyroxene and felspar 
ophitically and is evidently of comparatively late crystallization. 
Interstitial Material Between the other minerals, the interstitial 
spaces, sometimes larger than the average grain-size of the rock, but 
