Lhe Wackerfield Dyke. 47 
Microscopic.—Under the microscope all the fresh rocks have 
precisely similar characters, the only detectable variation being in 
the quantitative proportions of the constituents. The minerals 
may be considered in four groups: plagioclase, pyroxenes, black 
ores, and interstitial material. No glass is present, but the inter- 
stitial material, which appears to be mainly quartz, occupies many 
of the wedged-shaped spaces between the essential constituents, 
and the dominant texture may therefore be described as intersertal. 
Ophitic relations are also developed in places, though not con- 
spicuously, The characteristic appearance seen in thin section is 
illustrated by Fig. 1 of Pl. VIII. 
Plagioclase.—As in the Whin Sill plagioclase is the most abundant 
constituent. Kxcept in the marginal facies of the rocks it is 
perfectly fresh, generally free from inclusions and of prismatic habit, 
giving lath-shaped and nearly square sections. The smaller prisms - 
are of uniform composition, but larger crystals of the same habit 
show zoning, while the rare phenocrysts are again uniform but 
markedly calcic in composition. The smaller prisms constitute the 
dominant type, and the extinction angles, average refractive index, 
and specific gravity indicate that its composition is not far from 
Ab Ang. The larger prisms are richer in anorthite in the core, 
which is often well marked, but the outer layers vary rapidly from 
labradorite to oligoclase, the peripheral parts having practically 
the same refractive index as Canada balsam. The range is thus from 
about Abs) Anz to nearly Abo) Any. The rare phenocrysts all show 
irregularly rounded forms suggesting resorption of the original 
peripheral portions, The internal parts approach anorthite in 
composition (about Abio Ang), and there is little change towards 
the corroded rim. Irregular streaky inclusions are present as in the 
larger felspars of the Whin Sill and the corroded and _ dislocated 
felspar aggregates of the Crookdene and Collywell dykes of 
Northumberland.t » Viewed under a high-power objective, the 
inclusions are found to consist of pyroxene, felspar, and black ores, 
suggesting that they were originally blebs of magma which after- 
wards consolidated inside the felspars which caught them up. 
Miss Heslop and Dr. Smythe suggest that the anorthite aggregates 
of the Crookdene and Collywell dykes were formed in depth and 
that they floated up through the magma and thus tended to become 
concentrated near the head of the dykes, where, in fact, they are 
found. Viewed in this light, the observations point to the present 
exposure of the Morpeth Dyke having been the lowest, that is, the 
farthest removed from the head of the dyke at the time of con- 
solidation ; next, in ascending order, comes the Tynemouth Dyke, 
and, lastly, the dykes of Collywell and Crookdene.” 2 Although 
the Wackerfield dyke differs in certain respects from those investi- 
gated by Miss Heslop and Dr. Smythe, it appears likely that the 
1M. K. Heslop & J. A. Smythe, O.J.G.8., vol. Ixvi, 1910, p. 7. 
ay Weptol ss yoy lr(s 
