PART I. CHAPTER VIII. 



109 



Rocks altered by Trap Dikes. 



character of coke ; while those close to it are converted into a 

 substance resembling soot.* 



As examples might be multiplied without end, I shall merely- 

 select one or two others, and then conclude. The rock of Stir- 

 ling Castle is a calcareous sandstone, fractured, and forcibly dis- 

 placed by a mass of greenstone, which has evidently invaded 

 the strata in a melted state. The sandstone has been indurated, 

 and has assumed a texture approaching to hornstone near the 

 junction. In Arthur's Seat and Salisbury Craig, near Edin- 

 burgh, a sandstone which comes in contact with greenstone, is 

 converted into a jaspideous rock.f 



The secondary sandstones in Sky are converted into solid 

 quartz in several places, where they come in contact with veins 

 or masses of trap ; and a bed of quartz, says Dr. MacCuUoch, 

 found near a mass of trap, among the coal strata of Fife, was 

 in all probability a stratum of ordinary sandstone, having been 

 subsequently indurated and turned into quartzite by the action 

 of heat.J 



But although strata in the neighbourhood of dikes are thus 

 altered in a variety of cases, shale being turned into flinty slate 

 or jasper, limestone into crystalline marble, sandstone into 

 quartz, coal into coke, and the fossil remains of all such strata 

 wholly or in part obliterated, it is by no means uncommon to 

 meet with the same rocks, even in the same districts, absolutely 

 unchanged in the proximity of volcanic dikes. 



This great inequality in the effects of the igneous rocks may 

 often arise from an original difference in their temperature, and 

 in that of the entangled gases, such as is ascertained to prevail 

 in different lavas, or in the same lava near its source and at a 

 distance from it. The power also of the invaded rocks to con- 

 duct heat may vary, according to their composition, structure, 

 and the fractures which they may have experienced, and per- 

 haps, also, according to the quantity of water (so capable of 

 being heated) which they contain. It must happen in some 

 cases that the component materials are mixed in such propor- 

 tions as prepare them readily to enter into chemical union, and 

 form new minerals ; while in other cases the mass may be more 

 homogeneous, or the proportions less adapted for such union. 



We must also take into consideration, that one fissure may be 

 simply filled with lava, which may begin to cool from the first ; 



* Sedgwick, Camb. Trans., vol. ii, p. 37. 



t niust. of Hutt. Theory, § 253. and Dr. MacCuiloch, Geol. Trans., First 

 Series, vol. ii. p. 305. 



t Syst. of Geol., vol. i. p. 206. 

 K 



