﻿Vol. 66. ,] DYKE AT CROOKDENE (NORTHUMBERLAND). 17 



which all the dykes have in common, suggests that they have been 

 derived either from the same source, or from pockets of molten 

 matter similar in composition and existing under similar conditions. 

 If the first hypothesis (as the simpler) be provisionally accepted , 

 then the observed differences among the dykes and the association 

 of phenomena may throw some light upon their history. Among 

 the most suggestive facts bearing on the subject we may note- 

 that those dykes which in the field can be shown to come to a 

 head are characterized by curved augites, large inclusions of 

 feebly-zoned anorthite, and chilled basalt in contact with these 

 inclusions. On the other hand, those dykes, the present exposures 

 of which are clearly below the natural top or head, are distinguished 

 by sharp-edged definite crystals, and scattered, corroded, and deeply- 

 zoned felspathie inclusions. Again, the most coarsely-grained rock 

 contains much olivine, few porphyritic felspars, and has its gas- 

 cavities completely filled by glass. 



These facts point to variation in physical conditions as the main 

 cause of the differences in rocks which have had, possibly, a 

 common origin. The peculiarities of the Collywell and Crookdene 

 rocks would seem to be explicable as the results of the rapid fall 

 of temperature consequent on their position at the top of the dyke. 

 Opportunity was not afforded for the complete development of the 

 ground-mass crystals owing to rapid cooling ; possibly, too, diffusion 

 was checked by the great viscosity of the medium, and the molten 

 rock enveloping the felspar inclusions could furnish but little 

 material for their growth before consolidation set in. The results 

 are the stunting and distortion of the crystals of the ground-mass 

 and the feeble zoning of the felspar inclusions. 



The Morpeth and Tynemouth rocks would seem to have 

 consolidated lower down in the dykes where cooling was slower,, 

 the magma more fluid, and thus the conditions more favourable for 

 the growth of the ground-mass crystals. The felspathie inclusions 

 (smaller and more scattered in proportion to their distance from 

 the top of the dyke) had time to enable them to add considerably 

 to their substance, so that zoning is well developed. Increased 

 pressure, too, may have been a factor of importance, favouring the 

 formation of a different mineral species, olivine, from a magma 

 which, in other circumstances, would only develop augite. Again, 

 when crystallization had reached an advanced stage, and cooling- 

 had proceeded so far that the gaseous matter in the rock had lost 

 its elasticity, the crystallization-residues were evidently still fluid 

 enough in many cases to fill completely the now vacuous gas- 

 cavities. 



Viewed in this light, the observations point to the present 

 exposure of the Morpeth Dyke having been the lowest, that is, 

 the farthest removed from the head of the dyke, at the time of 

 consolidation ; next, in ascending order, comes the Tynemouth 

 Dyke ; and, lastly, the dykes of Collywell and Crookdene. 



Q. J. G. S. No. 261. c 



