The dykes of basalt or greenstone, that intersect the coal measures, are among the most remarkable occurrences in the Coalfield.
The most considerable basaltic dyke in the immediate neighbourhood of Newcastle is that which passes through Coley hill, about 4 miles west of the town. A long range of quarries has here been opened upon it, in some places to the depth of 50 feet, and laying bare the entire width of the dyke, which is 24 feet. The dyke in this place appears to have no hade. The basalt of which it is composed is found in detached masses coated with yellow ochre. The removal of these brings to view thin layers of indurated clay with which the fissure is lined, and which breaking into small quadrangular prisms is used by the country people for whetstones: in this substance clay ironstone impressed with the figures of ferns is very abundant. The upper seam of coal is here found at about 35 feet from the surface, and where in contact with the dyke is completely charred, forming an ash-grey porous mass, which breaks into small columnar concretions, exactly resembling the coak obtained by baking coal in close iron cylinders in the process of distilling coal-tar. Calcareous spar and sulphur are disseminated through the pores of this substance.
The basalt itself when broken is of a greenish-black colour, and of a coarse grained fracture. It contains quartz, calcareous spar, and another mineral, possessing the following characters. The colour is wax-yellow passing into olive-green; the lustre vitreous, resembling that of glassy felspar; the fracture foliated. It resists the action of the blow-pipe without borax, but with it melts into a white glass. The latter circumstance, and the foliated fracture distinguish this substance from olivine, which gives a dark green bead with borax, and presents a fracture more or less conchoidal.
Passing to the east-south-east of the Coley hill dyke in the line
