BASALTIC DIKES. 



123 



Where we find basalt presenting a perpendicular 

 precipice, as at the Palisades, it is very probable 

 that it has been injected through strata of soft ma- 

 terials, such as shale or tufa, which, being more 

 perishable than the trap, have been v^ashed away 

 by the sea, rivers, or rain, leaving the dike standing 

 out in the form of a precipice, thus : 



Rocks altered by volcanic dikes. — It might be ex- 

 pected that rocks thrown up in a melted state, 

 through fissures and crevices in other rocks, would 

 produce material alterations in those portions lying 

 nearest to the heated mass, and such we find to be 

 the case. In the north part of Ireland, a bed of 

 chalk 270 feet thick is traversed by dikes of basalt ; 

 and at the line of contact, and at several feet from 

 the wall of the dike, the chalk is changed into a 

 dark brown, crystalline rock, the crystals running 

 in flakes, like those of coarse primitive limestone. 

 To this succeeds a still finer-grained variety, and 

 by degrees it becomes compact, with a porcellane- 

 ous aspect and a bluish-gray colour, till finally it 

 becomes of a yellowish-white colour, and passes in- 

 sensibly into unaltered chalk. The flints contain- 

 ed in the indurated part of the chalk are of a yel- 



and regular forms of basaltic columns havo resulted from the 

 crystalline arrangement of the particles in cooling ; and the con- 

 cavities or sockets have been formed by one set of prisms press* 

 ing upon others, and occasioning the upper spheres to sink into 

 those beneath, and thus the different layers have been articula- 

 ted together. It is not, however, to be inferred that basalt al 

 ways assumes this shape. 



Fig. 34. 



