BASALTIC DIKES. 
123 
Wh^re 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 washed away 
by the sea, rivers, or rain, leaving the dike standing 
out in the form of a precipice, thus : 
Fig. 34. 
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 have 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. 
