108 Miss M. K. Heslop—Pre-Tertiary Dyke, Usway Burn. 
Tar Guassy Bast. 
With low powers it looks as though the porphyritic and ground-mass 
crystals were embedded in an almost homogeneous dark-brown’ 
background ; but when the latter is examined with higher powers, 
it appears that the ultimate base is colourless and contains numbers 
of brown patches which give the characteristic colour. These patches 
have borders of a yet darker hue (see Fig. 4, Pl. XI), and from their 
corners send out processes into the clear base. The highest powers, 
cannot resolve the brown areas into any constituent part, but if may 
readily be seen that the dark edges are due to numbers of globulites 
which also form the processes, being carried out apparently on a central 
thread of material from the patch. 
The dark areas show a marked tendency towards angularity of 
shape, many are rectangular, while others have more hexagonal 
outlines. They are considerably deformed by the protrusion of the 
globulitic processes, which, besides obliterating the corners, curve 
the sides. Some are quite shapeless. The colour of the glass is 
greatly intensified by numbers of small reddish-brown hexagons of 
a micaceous oxide of iron, probably hematite (see Fig. 4 in Plate). 
These little plates are never very thick, for sections perpendicular 
to them always give quite narrow dark needles. Irregular grains 
of a black oxide of iron are fairly common.’ They have never been 
known to show any definite crystalline form, and are probably in 
a very elementary stage of development. Crystals of the iron 
oxides, like those of the pyroxene (see Fig. 5, Pl. XI), are sur- 
rounded by light-coloured areas when embedded in a dark patch. 
Apatite is repeated in this generation in small but very perfect 
needles, which pierce all the other crystals and are certainly older 
than most of them, although the precise relation of their age to 
that of the apatites of earlier periods of crystallization is somewhat 
obscure. The early felspars are of the same type as those of the 
ground-mass. The crystals in this case, however, are only solid at 
the centre, and grow in hollow tubes in both directions, con- 
sequently longitudinal sections show the well-known ‘hour-glass’ 
structure (see Fig. 3, Pl. XI), while cross-sections of any but the 
central part show square colourless. frames of felspar material filled 
with glass. No globulitic stage of felspar growth has yet been seen, 
the very earliest recognizable forms being faint colourless streaks, 
only visible when surrounded by dark glass. 
The simplest elementary pyroxenes are short cigar-shaped prisms 
which occur either in pairs or groups, rarely alone. All possible 
combinations of these have been observed. ‘he pairs are attached 
at their centres, while the ends of both curve away from each other, 
thus imitating the ‘hour-glass’ structure of the felspars. Some 
assume the usual skeleton form, consisting of a central stem with 
little perpendicular arms attached on either side; others adopt the 
curving acanthus-like structure which is so strongly developed in the 
Collywell and Crookdene Dykes of Northumberland.' 
1 « Notes on the Crookdene and Related Dykes,’ by M. K. Heslop, M.Sc., and 
Dr. J. A. Smythe, read before the Geological Society, November 17, 1909. 
