Miss WM. K. Heslop—Pre-Tertiary Dyke, Usway Burn. 109 
The largest elementary pyroxenes are prismatic in shape, and they 
appear to grow by the addition of material at the corners, and it is 
remurkable that quite detached particles are often arranged i” line 
with each other and with the edge of a small pyroxene, for consider- 
able distances. In one case a ‘ process’, about ‘0375 mm. in length, 
protrudes from the end of a small prism of pyroxene, and is made up 
of several apparently quite detached particles. ‘The intervals between 
them are too minute to be correctly estimated—it is quite evident, 
however, that each piece is separate. 
This kind of growth seems to prevail among the elementary pyroxenes, 
because, not only are there innumerable examples of it, but in the 
more advanced crystals cross-sections with good octagonal forms and 
sharp angles often show a gap filled with glass running across the 
centre of the section almost at right angles to the pinacoids (see 
Figs. 5 and 6, Pl. XI). If we refer again to the simplest elementary 
pyroxenes, those consisting of two cigar-shaped needles joined at the 
centre, but with tapering ends diverging apart, we see that there is 
a tendency for pyroxene to grow in incomplete, perhaps hollow, prisms 
from a solid centre, as in the case of the ‘ hour-glass’ felspars. This, 
no doubt gives rise to the centrally mcomplete sections which have 
just been described, and to the intergrowth of pyroxene with the pale 
yellow material—again in incomplete crystals—which occurs in the 
ground-mass generation (see description of ground-mass pyroxenes). 
The gaps are always filled by this pale yellow, feebly doubly- 
refracting substance, which seems indeed to have been brought almost 
to a erystallizable state by the subtraction of pyroxene material to 
build the crystals of that mineral. 
In one slide the glass is in a much more developed stage. The base 
is still colourless, but there is a marked increase in the number of 
elementary crystals, especially the iron oxides and the little pyroxene 
needles. The brown patches have practically disappeared, but their 
final stages may be recognized in the small clouds of dark granules 
which are usually associated with elementary pyroxenes. 
A practically unbroken transition from the dark granules to definitely 
hexagonal plates of micaceous iron oxide has been made out. With 
a magnification of 1000 diameters, very small granules may be seen 
to possess hexagonal outlines, and although many may belong to the 
black oxide, there can be little doubt that the brown patches of the 
normal glass are here very largely represented by hexagons of micaceous 
oxide of iron. 
In the same way, and with equal certainty, a transition from the 
globulitic processes of the brown patches to the elementary cigar- 
shaped pyroxenes may be traced. These may be certainly identified 
by their interference colours, needles with a thickness of less than 
00236 mm., giving quite recognizable interference tints. It is 
impossible to say definitely whether these early forms are orthorhombic 
or monoclinic, but the number of cases in which the extinction is 
oblique suggests the latter. 
The association of pyroxene and iron oxide in the brown patches is 
similar to the association of the basic elements, augite and iron oxide, 
in the ‘ basic globulite’ of the post-Tertiary Dykes of Northumberland. 
