180 THE BASALTIC DYKE. 



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 Lilhoue cross, it ranges along the moors like an ancient 

 military road ; but in a large portion of its range, especially in the wide 

 vale of Tees, it is concealed by diluvial accumulations. 







The composition of the basalt presents few peculiarities. Olivine, 

 calcareous spar and quartz* are the principal extraneous minerals. Hollow 

 geodes occur in it, of which the walls are amethystine quartz, presenting 

 crystalline facets to the cavity which contains a crystal of carbonate 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 the 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, irregular, 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 ; sandstone, shale, and coal ; new red sandstone and red 

 marl ; lias shale, &c. and the lower sandstones of the oolitic series. These 

 strata, where they come in contact with the basalt, are more or less 

 altered in appearance 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, and 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 



* Mr. D. Tuke, in a communication to the Yorkshire Philosophical Society. 



