. The elements of geology, for popular use : containing a description of the geological formations and mineral resources of the United States. Geology; Geology. 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 washed away by the sea, rivers, or rain, leaving the dike standing out in the form of a precipice, thus : Fig. Rocks altered hy volcanic dikes.—It might be ex- pected that roc
. The elements of geology, for popular use : containing a description of the geological formations and mineral resources of the United States. Geology; Geology. 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 washed away by the sea, rivers, or rain, leaving the dike standing out in the form of a precipice, thus : Fig. Rocks altered hy 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 I 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 Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations ma
Size: 2820px × 886px
Photo credit: © Paul Fearn / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookpublishernewyorkharperb, booksubjectgeology
