SOME NORTH-OF-ENGLAND DYKES. 235 
Mag. decade ii. vol. x. p. 252 ; but they are again inserted here in 
order to make the description of the Tynemouth rock complete. 
The spherical amygdaloids, like the porphyritic crystals, are very 
abundant in certain specimens and almost entirely absent in others ; 
they vary in size from that of a mustard-seed to that of a pepper- 
corn, and, as a general rule, where the amygdaloids are most nume- 
rous the porphyritic crystals are least numerous, and vice versa. 
The amygdules effervesce freely with hydrochloric acid and are thus 
seen to be largely composed of calcite. 
The ground-mass of the rock in which the above elements occur 
is crystalline in texture and of a very dark colour. Fractured sur- 
faces are somewhat uneven. A bulk-analysis of the rock yielded 
to Mr. Stead the following result :— 
SUG alae ipods abicn. dtch ee) seo DOR OU 
PAU TIMI afc o 5 hclis espns a eens 16°14 
IHerrous OmIdes.-). aan eee 4-50 
MMELTICHORUGMS Ws). i, <n os 4-76 
TEAS) OSS anemone se eee 10-96 
IGRI 99s Se cee ges cone 2°68 
mobashiyss tab) cis cel re 80294 
ROO a de ete ee 1-74 
100-02 
Three separate determinations of the density gave 2°344, 2°845, 
2°839. 
Under the microscope (Pl. XIII. fig. 1) the ground-mass is seen to 
be identical with that of the Hebburn dyke; it is therefore unne- 
cessary to describe it in detail. In two out of the seven slides ex- 
amined the serpentinous mineral appears to form a pseudomorph after 
olivine; and we may conclude, I think, that olivine was sparingly 
present in the original rock. ‘The large porphyritic crystals call for 
more detailed description. Under polarized light they are at once 
seen to consist, as a rule, of crystalline granular aggregates, as many 
as 7 or 8 crystalline grains going to make up one of the larger porphy- 
ritic masses. The different individuals in a complex mass are not 
orientated in any definite manner with regard to each other, nor 
do the interior boundaries of the crystalline particles show definite 
faces, although the external boundaries usually do. Another feature 
of great interest is to be found in the fact that a narrow peripheral 
zone of felspar substance is usually present, which extinguishes at 
a different angle from the central kernel, and that it is this peri- 
pheral zone which gives the definite crystalline form to many of the 
grains. I have never seen this zone continued between two indivi- 
dual grains of one and the same aggregate. 
It appears, then, that granular aggregates of anorthite were first 
formed without external crystalline faces, and that, during the later 
stages of consolidation, additional felspar substance of somewhat 
different composition was added, so as to give definite form to the 
