﻿32 MISS M. K. HESLOP AND DK. J. A. SMYTHE ON THE [Feb. I9IO, 



traversed by broad cracks, which are filled with small fragments of 

 felspar embedded in a ground-mass of decomposed felspar dust. 

 This dust forms the base in which all the crystals are embedded 

 and by which they are bound together ; it is occasionally pene- 

 trated by threads of greenish glass. In one case, some of the larger 

 cracks in the felspar are invaded by tongues of basalt containing 

 small felspars and augites. The basalt in contact with this 

 segregation is, apparently, slightly chilled, the ground-mass crystals 

 at the junction being much smaller than usual (fig. 2, p. 11). The 

 change is perfectly abrupt ; there is no concentration of felspar in 

 the basalt near the aggregate ; and this, together with the penetra- 

 tion of basalt along the cracks, shows that these felspar masses are 

 inclusions older than the ground-mass generation. 



The close resemblance between the dykes at Collywell and 

 Crookdene in chemical composition and in the structure of the 

 rocks and their anorthite inclusions — a resemblance which almost 

 amounts to complete identity — suggests the possibility of their being 

 different exposures of the same intrusion. Both dykes run west by 

 north to east by south ; the distance between them is 24 miles. 

 The line joining the two passes through what are usually regarded 

 as exposures of the Collywell Dyke in Holywell Dene, close to Colly- 

 well, and Shankhouse Pit, a few miles inland. A little farther 

 west, at Middle Duddoe, it passes near to a small exposure of a dyke, 

 marked on the Survey maps but now no longer visible. It seems 

 likely, then, in view of these facts, that these five exposures of 

 basalt all belong to the same intrusion. It may be mentioned 

 that the faults in the neighbourhood run chiefly east and west, and 

 so would not be likely to shift the outcrop of this dyke. A great 

 part of the country through which the dyke cuts is covered with 

 drift ; there are, nevertheless, frequent opportunities for the dyke 

 to come to light, and that it does so but rarely may perhaps be 

 ascribed to the fact that, as at Collywell and probably just east 

 of Crookdene, it fails to reach the surface. 



III. The Tynemoeth Dyke. 



This dyke has been fully described by Dr. Teall, 1 and therefore 

 a brief description of it will suffice in this place. 



On reference to the analysis (p. 3) it will be noted that this 

 rock is somewhat more acidic than the other dykes. Microscopic 

 examination shows that it contains also more residual glass than 

 these. It seems possible that these facts are interdependent.' 2 



It is a fairly even-grained rock, composed of felspar, augite, and 

 some crystals of iron oxide. The felspars are in well-defined laths 

 which show binary twinning, and not infrequently a stellate arrange- 



1 Quart, Journ. Geol. Soc. vol. xl (1884) pp. 238-36; also Geol. Mag. 

 dec. iii, vol. vi (1889) pp. 481-83. 



2 See J. J. H. Teall, 'British Petrography' 1888, pp. 42. 43, 



