﻿Vol. 66.] DYKE AT CROOKDENE (NORTHUMBERLAND). 9 



Consideration of all these facts leaves little doubt that the felspar 

 aggregates are inclusions, and it seems most likely that they are 

 inclusions of an earlier crystallization in a later. The absence of 

 crystalline outlines in the interior of the aggregates, and the occur- 

 rence of dark inclusions of glass in the crystals, are strongly 

 suggestive of crystallization from a fairly pure felspathic magma 

 which had separated from a more basic one, the slight excess of 

 basic material being eliminated during the crystallization of the 

 felspar. That this basic material contains augite and iron oxide is 

 further suggestive that the inclusions as a whole separated out at 

 some early stage from the basalt with which they are associated. 

 There is thus evidence of differentiation of the magma from which 

 the dyke is derived, such differentiation, possibly gravitational, 

 resulting in the production of pockets or crusts (presumably speci- 

 fically lighter) of solid material approaching anorthite in composition. 



If such differentiation took place prior to intrusion, then on 

 intrusion the crusts would be broken up and floated away by the 

 molten rock, and would tend to aggregate near the head of the 

 dyke ; possibly, too, the felspar crystals would be unstable by reason 

 of the changed conditions, especially of pressure, and would tend 

 to disruption by release of internal strain, somewhat after the 

 manner of a Rupert's drop. As these felspathic inclusions are 

 embedded in molten matter containing the constituents necessary 

 for their growth, a certain amount of fresh felspathic material 

 would be added to them at their contact with the basalt, producing 

 thus the zoning which is characteristic of this contact. 



II. The Colltwell Dyke. 



This dyke has been briefly described by Dr. Teall 1 under 

 the name of the Hartley Dyke, but as it is always termed the 

 Colly well Dyke locally, it seems well to retain that name. It is 

 well exposed at present (1909) on the coast about half a mile north 

 of Hartley, and is seen to come to a natural head. Specimens of the 

 altered dyke at Shankhouse Pit, a few miles inland, were examined 

 by Teall and found to contain fresh felspar, but to have the pyroxene 

 replaced by calcite. The effect of weathering is thus the same as 

 that which we have observed in the case of the Crookdene Dyke. 

 Near the top of the exposure at Collywell the rock is much decom- 

 posed, and effervesces freely with hydrochloric acid ; lower down, 

 however, it is hard and fresh. Specimens of this fresh rock were 

 analysed, with results already stated (p. 3). 



In chemical composition there is the very closest resemblance 

 between this rock and the Crookdene Dyke ; and sections under the 

 microscope show the same general structure, although the rock is, 

 perhaps, rather finer-grained and contains a little more unindi- 

 vidualized iron oxide in the glass. 



The felspars are long and narrow, and the augite is wrapped 



1 Quart. Journ. Geol. Soc. vol. xl (1884) p. 237. 



