482 J. J. H. Teall—Amygdaloids of Tynemouth Dyke. 
under crossed Nicols reveals the fact that the felspar-substance to 
which the external idiomorphism is due differs from that forming 
the central portions. This, taken in connection with the fact that 
the augite-grains of the ground-mass are occasionally included in the 
peripheral zone of the porphyritic groups, justifies the conclusion 
that such external form as the individuals possess was given to them 
at a later stage in the history of the consolidation of the rock than 
that at which the groups themselves were formed, and also under 
different physical conditions. 
The augite is pale in colour, and occurs in grains or granular 
aggregates. It is occasionally penetrated by the lath-shaped felspars, 
and must, on the whole, have been formed after them. 
The lath-shaped felspars call for no special description. They 
frequently show multiple twinning of the usual type. 
The interstitial matter contains extremely minute microlites and 
skeleton crystals of felspar, grains and skeletons of magnetite, and 
an indistinct brownish granular substance. It is not possible to 
recognize any true glass, even with the highest powers. ‘This 
interstitial matter occurs in more or less angular patches wedged in 
between the other constituents. and gives to the rock the structure 
for which Professor Rosenbusch has proposed the term intersertal. 
The rock itself would be termed by this author a tholeite. 
Now the peculiar feature to which I wish to call special attention 
is the occasional occurrence of spherical patches of interstitial matter. 
These appear in the thin sections—and they have only been recog- 
nized in the sections—as circles. How are these spherical patches to 
be accounted for? An answer to this question is found by studying ~ 
the amygdaloids, which have been already referred to as occurring 
in certain portions of the dyke. (See Pl. XIV. Fig. 2.) 
Microscopic examination enables us to determine the precise stage 
in the history of consolidation at which the vesicular cavities, 
now for the most part filled with carbonates with or without a 
narrow border of chalcedony, were formed. Their development 
evidently displaced the lath-shaped felspars, for these are often 
arranged tangentially with reference to the bubbles; but it produced 
no efiect on the disposition of the constituents of the interstitial 
matter. It appears, then, that the gas bubbles were produced after 
the formation of the porphyritic constituents, the augite and the lath- 
shaped felspars, but before the consolidation of the interstitial matter. 
It is possible that their development was due to the relief of pressure 
consequent on the rise of the semi-liquid mass in the crack. If so, 
then they are analogous, so far as their mode of formation is con- 
cerned, to the bubbles which arise in the contents of a soda-water 
bottle as the cork is partially removed. 
Now, the spherical patches of interstitial matter agree in form and 
size with the amygdaloids, and to account for them we have only to 
suppose that the portion of the mass which was liquid at the time of 
their formation, oozed into some of the vesicles owing to the absorp- 
tion, escape, or condensation of the gas. That this is the true expla- 
nation is proved by the occurrence of cavities which have been only 
