232 J. J. H. TEALL—PETROLOGICAL NOTES ON 
the Hebburn dyke is in Boldon pit. To this exposure I was con- 
ducted through the kindness of Mr. Walker, who also supplied me 
with a sketch from which the diagram (fig. 2), expressing the 
relation of the whinstone to the Coal-measures, is constructed. 
The section crosses the dyke about 22° from a right angle. 
The actual thickness of the dyke in the “blue metal” is thus 
seen to be 44’ 6”, and in the coal 49’. 
Macroscopically the rock is dark, almost black in colour and 
uniformly crystalline in texture, 7. ¢. porphyritic elements are 
wanting. At the margins itis compact. In the central parts it 
effervesces slightly, and in the marginal parts freely with hydro- 
chloric acid. Here and there occur small spherical amygdaloids, 
measuring on the average about 1 mm. in diameter; these are 
occupied by calcite and quartz. The specific gravity of the un- 
altered rock is 2°84. 
Under the microscope the rock is seen to consist of magnetite 
(? ilmenite), felspar, pyroxene, and a considerable quantity of inter- 
stitial matter, which has a decided though indefinite reaction on 
polarized light. This ground-paste is rendered in places almost 
opaque by a large quantity of brown granular matter. It also 
contains skeleton felspars and acicular microlites of felspar, which 
do not always appear to be sharply separated from the paste. 
The felspar sections are usually very long in proportion to their 
width, a feature which characterizes many of the dykes now about 
to be described (Pl. XII. fig.6). Binary twins are most numerous. 
The pyroxene is almost colourless, frequently twinned and evidently 
monoclinic ; it occurs in irregular plates of secondary consolidation 
and rarely, if ever, shows definite crystalline boundaries in those sec- 
tions which are taken from the main mass of the dyke. 
The mutual relation of the felspar and pyroxene is that so 
eminently characteristic of the German and Swedish diabases, but 
which is also found in certain typical dolerites. 
The secondary products are calcite, quartz, and a greenish serpen- 
tinous mineral giving aggregate polarization. The latter mineral 
occurs sparingly, and may possibly be a pseudomorph after olivine. 
Calcite and quartz occur in the spherical amygdaloids already 
mentioned, the former mineral making up the central and larger 
portion of the amygdule, and the latter occurring as detached crystals 
or crystalline granular aggregates near the periphery. It is especially 
worthy of note that the lath-shaped felspar sections are frequently 
arranged with their longer axes parallel to the boundaries of the 
amygdaloids, a fact which can only be explained by supposing that 
the cavities were produced when the rock was plastic. The presence 
of gas-cavities in a rock which was evidently produced under great 
pressure is somewhat anomalous. 
The marginal portion differs in texture from the main mass of the 
dyke. It is micro-porphyritic ; but, owing to the enormous amount 
of alteration which has taken place, it is somewhat difficult to make 
out its original character. The larger constituents are irregular 
grains of magnetite, lath-shaped felspars, frequently having bifid 
