Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 2.djvu/23

 Rh the form of Dikes, filled with materials similar to those which form the masses that have overflowed in the Vicinity of each Dike.

The peculiar condition of the rocks that form the side walls of Granitic Veins and Basaltic Dikes, affords another argument in favour of their igneous origin; thus wherever the early Slate rocks are intersected by Granitic Veins (a. 8.) they are usually altered to a state approximating to that of fine-grained Mica-slate, or Hornblende-slate.

The secondary and Tertiary rocks also, when they are intersected by basaltic Dikes, have frequently undergone some change; beds of Shale and Sandstone are indurated, and reduced to Jasper; compact Limestone and Chalk are converted to crystalline Marble, and Chalk-flints altered to a state like that resulting from heat in an artificial furnace.

In all these cases, the Phenomena appear to be throughout consistent with the theory of igneous Injection, and to be incapable of explanation on any other Hypothesis that has been proposed. A summary statement of the probable relations of the Granitic and Trappean Rocks to the other materials of the Globe, and to one another, may be found in De la Beche's Geological Researches, 1st Edit. Pag. 374, et seq.