Northumberland and Durham. 21 



The dykes of basalt or greenstone, that intersect the coal mea- 

 sures, are among the most remarkable occurrences in the Coalfield. 



The most considerable basaltic dyke in the immediate neigh- 

 bourhood 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 break- 

 ing 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 con- 

 tact with the dyke is completely charred, forming an ash-grey 

 porous mass, which breaks into small columnar concretions, ex- 

 actly 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 



