SOME NORTH-OF-ENGLAND DYKES. 229 
7] 
there small spherical amygdaloids, for the most part filled with 
calcite, may be recognized. The texture of the rock does not vary 
in any marked manner, the individual constituents being, as a rule, 
just recognizable by the naked eye. On applying acid, a slight 
effervescence may frequently be observed, and this naturally becomes 
more marked near the edges of the dyke and in the neighbourhood 
of joint-planes. 
Under the microscope the rock is seen to consist for the most 
part of felspar, pyroxene, and magnetite. Theres, however, also 
a small quantity of cryptocrystalline matter (Pl. XII. fig. 5). 
Felspar.—This occurs in forms which give lath-shaped sections, 
whose dimensions lie, as a rule, between the following limits :— 
08 x°3 millim. and :02x-14millim. The length of a section is, on 
the average, about five times its width. I am not able to recognize | 
more than one generation of felspar, so that the rock would be 
granular in the sense in which that term is used by Prof. Rosen- 
busch *. Under crossed Nicols the sections are seen to consist of 
two or more lamelle; but simple individuals are by no means un- 
common. Whenthe thin sections are exposed for some hours to the 
action of hydrochloric acid at a temperature of 100° C. the felspars 
lose their individual action on polarized light; and I conclude from 
this, as well as from the bulk-analysis of the rock, that they belong 
to a species allied to labradorite. 
Pyroxene.—This mineral rarely, if ever, presents definite crystal- 
line boundaries. It occurs in isolated grains, granular aggregates, 
and irregular plates, which are frequently interpenetrated by the 
lath-shaped felspars. There is no doubt, therefore, that it consoli- 
dated after the felspar. The prismatic cleavages are the only ones 
that I have observed. Twinning is not uncommon. 
Magnetite occurs in grains and crystals scattered uniformly 
through the rock, the sections measuring about :12 or -1 millim. 
across. That it is magnetite and not ilmenite appears to be proved by 
its tendency to give rise to limonite decomposition-products, and by 
the readiness with which it is attacked or removed by hot hydro- 
chloric acid. 
The elements already described make up perhaps nine tenths of 
the mass. As already stated, however, there occurs also a small 
quantity of interstitial matter which gives an indistinct reaction 
under crossed Nicols; it is brown in colour, granular in aspect, and 
usually charged with minute colourless acicular microlites. This 
doubtless represents the highly acid residue which remained after 
the separation of the definitely crystalline constituents. The secon- 
dary minerals are calcite and quartz. Green alteration- powers 
are very rare in the specimens examined by myself. 
The specific gravity of the unaltered rock varies from 2°94 to 
2:96. Its chemical composition, as given by Mr. I. Lowthian Bell, 
F.R.S., is as follows :— 
* “Ueber das Wesen der kornigen und porphyrischen Structur bei Massen- 
gesteinen,’ Neues Jahrbuch (1882), Band ii. p. 1. 
