SOME NORTH-OF-ENGLAND DYKES. 937 
dark grey, or greenish-grey, finely crystalline rock, portions of which 
bear a very close resemblance to the ground-mass of the Tynemouth 
dyke. Porphyritic crystals of felspar are very rare, but not entirely 
absent. At the north-west margin there occurs a considerable 
thickness, perhaps 12 or 14 feet, of what appears to be highly altered 
whinstone. This varies in colour from a light grey to a dark greenish 
grey, with narrow zones of brown due to the segregation of iron 
oxides along bounding surfaces and joint-planes. The altered por- 
tion usually effervesces very freely with hydrochloric acid; and 
cracks and amygdaloidal cavities filled with calcite and chalcedony 
are very abundant. The total thickness of the dyke is about 20 or 
25 feet. 
Under the microscope the central and comparatively unaltered 
portions are seen to consist of long narrow lath-shaped felspars, 
irregular crystalline grains and plates of a nearly colourless pyroxene, 
and a small quantity of nearly opaque interstitial matter. The iron 
oxide does not seem to have separated out, but to have remained in 
solution in the paste until the final act of consolidation took place. 
This is probably only a local peculiarity (Pl. XII. fig. 6). The 
marginal portions of the dyke are so much altered that it is difficult 
to make out their original characters under the microscope. It is 
worthy of note, however, that the felspars are comparatively fresh 
even when the other constituents have lost all their original cha- 
racters. The amygdaloids and veins of calcite and chalcedony do 
not call for any special description. The specific gravity of the rock 
is 2°9. 
Tur SEATON AND Hartriey Dyxrs. 
Literature. 
Lzzovr, Prof.G. A. Geology of Northumberland, p. 48. 
Ordnance Maps 105 N.E. and N.W. 
In this district there are several parallel dykes running at short 
distances from each other in a direction slightly N. of W.and S. of E. 
They are exposed on the shore between Seaton and Hartley (one 
near the northern angle of the shore and another near the spring 
which rises on the shore), in the small valley to the west of the 
village, where they are now (1882) being quarried for road-metal, 
and also in the Shankhouse collieries further west. At page 48 of his 
‘Geology of Northumberland,’ Prof. Lebour writes :—“ In some cases 
the actual natural top or vertical dying-out of a dyke may be seen, 
as on the coast a little to the south of Seaton Sluice. Here, near 
a spring on the beach, a dyke, at least 12 feet in width where it 
rises at the foot of the cliff, is finely shown ending off in two tongues 
of basalt, the longest of which, after curving amongst disturbed and 
contorted shale, dies out about 11 feet from the ground beneath a 
bed of sandstone, which it had not the force to break through. The 
fault along which this dyke was injected is well seen continuing its 
upward course.” At the time of my visit this section was obscure, 
