Ch. XXIX.] STRUCTURE OF VOLCANIC ROCKS. (ft 5 



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 Stirling Castle is 

 a calcareous sandstone, fractured and forcibly displaced 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 ap- 

 proaching to hornstone near the junction. In Arthur's Seat and 

 Salisbury Craig, near Edinburgh, a sandstone which comes in contact 

 with greenstone is converted into a jaspideous rock. 



The secondary sandstones in Skye 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. MacCulloch, 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.f 



But although strata in the neighborhood 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 and in part 

 obliterated, it is by no means uncommon to meet with the same 

 rocks, even in the same districts, absolutely unchanged in the prox- 

 imity 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 conduct heat may vary, according 

 to their composition, structure, and the fractures which they may 

 have experienced, and perhaps, also, according to the quantity of 

 water (so capable of being heated) which they contain. It must hap- 

 pen in some cases that the component materials are mixed in such 

 proportions 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 sim- 

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

 in other cases the fissure may give passage to a current of melted 

 matter, which may ascend for days or months, feeding streams which 

 are overflowing the country above, or are ejected in the shape of sco- 

 riae from some crater. If the vails of a rent, moreover, are heated 

 by hot vapor before the lava rises, as we know may happen on the 

 flanks of a volcano, the additional caloric supplied by the dike and its 

 gases will act more powerfully. 



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

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



