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MISS M. K. HESLOP AND DR. J. A. SMYTHE ON THE [Feb. 19IO, 



Y. COMPARISON OE THE DYKES. 



The results of the examination of the four dykes may be summed 

 up conveniently in tabular form. As the differences between the 

 Collywell and Crookdene Dykes are very slight and unimportant, 

 those two may be taken together : — 



I. Collywell and 

 Crookdene. 



Fine, even-grained basalt com- 

 posed of felspar, angite and 

 iron oxide crystallized in the 

 order stated. 



Felspars occur in narrow laths, 

 around which augite is 

 wrapped in curved strands. 



Iron oxide in small skeleton 

 forms, not very plentiful. 



inclusions of anorthite 



Large 

 occur. 



Felspars of the inclusions 

 faintly zoned in contact 

 with the basalt only. In- 

 dividual crystals irregularly 

 intergrown, and rarely show- 

 ing good crystalline faces. 



Felspars of the inclusions 

 cracked, faulted, and shatter- 

 ed, and the basalt in contact 

 slightly modified (chilled). 



Basalt contains amygdaloids 

 of calcite, with dark residues. 



II. Tynemouth. 



As in I, but coarser-grained 

 and with some pseudomorphs 

 after olivine. 



Felspars larger than in I ; 

 augite gathered in around 

 them in" grains or in clear 

 straight crystals. 



Iron oxide mostly in unindivid- 

 ualized form darkening the 



glass. 



Porphyritic inclusions of an- 

 orthite in groups of two or 

 three, or in single crystals. 



Porphyritic felspars strongly 

 zoned, but a zone is never 

 carried between intergrown 

 crystals. Outer faces always 

 furnished by the zone, and 

 when the crystals are inter- 

 grown the boundary is 

 irregular. 



Porphyritic felspars crossed by 

 strain-bands giving brilliant 

 interference-colours. There 

 is no modification of the 

 basalt in contact with them. 



Basalt contains amygdaloids 

 of calcite and some of green 

 glass, both with crystalline 

 residues. 



III. MOKPETH. 



Coarser-grained than II ; 

 minerals as in I, with the 

 addition of olivine. 



Minerals as in II, with straight- 

 edged augites. 



Iron oxide as in II. 



Bare groups and single in- 

 dividuals of porphyritic 

 felspars (probably anorthite). 



Porphyritic felspars zoned as 

 in II. 



Porphyritic felspars show no 

 special characters, and the 

 basalt in contact is not 

 modified. 



Basalt contains amygdaloids 

 of green glass with dark 

 residues and no calcite. 



VI. Conclusions. 



Some conclusions suggested by the study of these dykes have 

 already been touched upon. We have seen reason, for instance, 

 for supposing that the Collywell and Crookdene Dykes are in 

 reality one, and that their felspathic aggregates are the result of 

 ma^matic differentiation under plutonic conditions. What applies 

 to these aggregates applies also to the porphyritic felspars of the 

 Tynemouth and Morpeth Dykes. This, and the many other points 



