THE 
GEOLOGICAL MAGAZINE. 
NEW SERIES. DECADE Ill. VOL. VI. 
No. XI.—_NOVEMBER, 1889. 
ORIGINAL ARTICIHS:- 
Pee ne ill 
J.—On tHE AMYGDALOIDS oF THE TynEmMouTH Dyxz.! 
By J. J. H. Tear, M.A., F.G.S: 
(PLATE XIV.) 
N a paper published in the Quarterly Journal of the Geological 
Society for 1884,? I gave some account of the Tynemouth Dyke. 
In that paper, however, I omitted to describe one feature, connected 
with the microscopic structure of the dyke, because at the time of 
writing I did not understand it. A short time ago I had occasion 
to re-examine my preparations, when my attention was again 
directed to the feature in question; and this time an explanation 
suggested itself which appears to be in every respect satisfactory. 
The main object of this communication is to supplement my already 
published description by giving an account of the feature to which 
I have referred, and which may be briefly described as the occurrence 
of spherical patches of interstitial matter. (See Pl. XLV. Fig. 1.) 
At the time of my residence at Tynemouth (1882) the dyke was 
exposed in the angle formed by the breakwater and the cliff on 
which the Priory stands, and also in the cutting close to the railway 
station. The rock of which the dyke is composed varies somewhat 
in character owing to the presence or absence of porphyritic felspars 
and small spherical amygdaloids. A typical specimen may be said 
to consist essentially of porphyritic crystals, or rather crystalline 
ageregates of a felspar closely allied to anorthite, embedded in a 
dark, finely-crystalline ground-mass, composed of augite, lath-shaped 
felspars, and interstitial matter. Olivine has been detected in one 
or two slides; but it is not usually present. 
The porphyritic constituents undoubtedly belong to the earliest 
phase in the consolidation of the original mass of molten matter. 
They consist, as a rule, not of single crystals, but of two or more 
individuals. Where the individuals of one and the same group are 
in contact with each other, they exhibit no trace of crystalline form ;? 
but where they are in contact with the ground-mass, they are bounded 
by definite faces. In other words, the internal relations of the 
individuals forming a group are those of plutonic rocks (e.g. gabbro), 
whereas the external relations of the same individuals are those of 
volcanic rocks. This, of course, is in strict accord with the general 
view that the porphyritic constituents have been developed under 
plutonic conditions. An examination of the porphyritic aggregates 
1 Read at the Brit. Assoc., Newcastle-on-Tyne, in Section C. (Geology), Sept. 1889. 
2 Petrological Notes on some North of England Dykes, vol. xl. p. 233. 
3 See fig. 1, plate xiii. accompanying the paper already referred to. 
DECADE III.—VOL. VI.—NO. XI. 31 
