EXPLANATION OF PLATE 1. 9 



the form of Dykes, filled with materials similar to those 

 which form the masses that have overflowed in the Vici- 

 nity of each Dyke.^ 



Changes effected hy the Igneous Rocks, on the Strata 

 in contact with them. 



The peculiar condition of the rocks that form the side 

 walls of Granitic Veins and Basaltic Dykes, affords ano- 

 ther 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 approxima- 

 ting to that of fine-grained Mica-Slate, Hornblende- 

 Slate. 



The Secondary and Tertiary rocks also, when they are 

 intersected by basaltic Dykes, 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 fur- 

 nace. f 



In all these cases, the Phenomena appear to be through- 

 out 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. 



* In many dykes the materials have been variously modified, by 

 their mode of cooling, and differ from the masses which overflowed 

 the surface. 



t Examples of this kind occur on the sides of Basaltic Dykes in- 

 tersecting Chalk in the County of Antrim, and in the Island of 

 Raghlin. See Geol. Trans. London, O. S. vol. iii. p. 210. pi. 10. 



