THE BASALTIC DYKE. 
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bound it, and, therefore, in some places it appears above them in a long 
crust or ridge. On Clifton rigg its blocks, lying bare on the surface, 
have been compared to prostrate pilasters half buried in ruins ; near 
Egton bridge it stands up in a lofty wall, over the waters of the Esk ; 
and beyond Silhoue cross, it ranges along the moors like an ancient 
military road ; but in a large portion of its course, especially in the wide 
vale of Tees, it is concealed by diluvial accumulations. 
The composition of the basalt presents few peculiarities. Olivine, cal- 
careous spar, mesotype, and quartz are the principal extraneous minerals. 
Hollow geodes occur in it, of which the walls are amethystine quartz, pre- 
senting crystalline facets to the cavity which contains a crystal of car- 
bonate of lime. The joints, which are often lined by a sooty substance, 
are in most quarries irregular, and lie in all directions ; but sometimes a 
tendency may be noticed to that horizontal prismatic structure, which 
prevails in narrower dykes of the same substance in the island of Arran. 
At Bolam, in Durham, where the mass extends itself more horizontally, 
the pseudo-prisms approach to a vertical position. Thin, flexuous, irre- 
gular, nearly horizontal layers of basalt appear in Langbargh quarry, and 
decomposing balls, with ochry outsides, are common. 
The following strata are divided by this remarkable dyke ; viz. 
mountain limestone near Middleton ; sandstone, shale, and coal, in Dur- 
ham ; magnesian limestone, new red sandstone, and red marl, in Durham 
and the north of Yorkshire ; lias shale, &c. and the lower sandstones of 
the oolitic series, in the north-eastern moorlands. These strata, where 
they come in contact with the basalt, are more or less altered in appear- 
ance and composition, and the change seems generally due to the action 
of heat. At Cockfield fell, the coal near the dyke is converted to a black 
substance like concreted soot, at a small distance changed to a cinder 
without bitumen or sulphur, and beyond, gradually regains its usual 
properties. “ In the stratum above the cinder a great deal of sulphur is 
sometimes found, in angular forms, of a bright yellow colour, and very 
beautiful.”* “ At Berwick on the Tees, the white sandstone is usually a 
* Mr. D. Tuke, in a communication to the Yorkshire Philosophical Society. 
