572 REPORT — 1889. 



crystalline structure of limestones associated with the crystalline schists appears to 

 bear a certain relation to that of the schists. To the latrer rule, however, exceptions 

 at first sight seem not infrequent, but microscopic examination shows that these 

 are due to the original coarser structure being to a great extent obliterated by 

 crushing. The well-known Tiree marble, where commonlj' rather large grains of 

 sahlite occur in a seemingly compact matrix of calcite, is found to be an instance 

 of the same, and microscopic examination shows that the whole rock was once 

 coarsely crystalline. The author concludes that, in limestones, pressure alone is a 

 comparatively minor agent in producing crystallisation, while it often renders 

 those which were once crystalline more fine-grained in structure. 



9. The Amygdaloids of the Tynemouth Dyke.^ 

 By J. J. H. Teall, M.A., F.G.8. 



In 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 witli 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 communica- 

 tion 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. 



At the time of my residence at Tynemouth (1832) 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 dj'ke is com- 

 posed varies somewhat in character owing to the presence or absence of porphy- 

 ritic felspars and small spherical amygdaloids. A typical specimen may be said 

 to consist essentially of porphyritic crystals, or rather crystalline aggregates of a 

 felspar closely allied to anorthite, embedded in a dark, finely crystalline ground- 

 mass, composed of augite, lath-shaped felspars, and interstitial matter. 



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 groundmass they are bounded by 

 definite faces. In other words, the internal relations of the individuals forming a 

 group are those of plutonic rocks {e.y., 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 porphyi'itic constituents have been developed under 

 plutonic conditions. An examination of the porphyritic aggregates 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 groundmass 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 diff'erent 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. 



' Published in frcol Mar/, for November 1880, pp. 481-483. 



- Pctrologioal Notrs on some North of England Dykes, vol. xl. 



3 See fig. 1, plate XIII., accompanying the paper already referred to. 



