370 ALTERATION OF STRATA 



[Ch. XXVI. 



traces of organic remains are effaced in that part of the lime- 

 stone which is most crystalline. 



As the carbonic acid has not been expelled, in'this instance, 

 from that part of the rock which must be supposed to have 

 been melted, the change must have taken place under consider- 

 able pressure ; for we know , by the experiments of Sir James 

 Hall, that it would require the weight of about 1700 feet of 

 sea-water, which would be equivalent to the pressure of a 

 column of liquid lava 600 feet high, to prevent this acid from 

 being given off. 



Another of the dikes of the north-east of Ireland has con- 

 verted a mass of red sandstone into hornstone *. By another, 

 the slate-clay of the coal-measures has been indurated, and has 

 assumed the character of flinty slate ( ; and in another place 

 the slate-clay of the lias has been changed into flinty slate, 

 which still retains numerous impressions of ammonites J. One 

 of the greenstone dikes of the same country passes through a 

 bed of coal, which it reduces to a cinder for the space of nine 

 feet on each side . 



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. Macculloch, 

 has been found near a mass of trap, among the coal-strata of 

 Fife, which was in all probability a stratum of ordinary sand- 

 stone subsequently indurated by the action of heat ||. 



Alterations of strata in contact with granite. Having 

 selected these from innumerable examples of mutations caused 

 by volcanic dikes, we may next consider the changes produced 

 by the contiguity of plutonic rocks. To some of these we 

 have already adverted, when speaking of granite veins, and 

 endeavouring to establish the igneous origin of granite. We 

 mentioned that the main body of the Cornish granite sends 

 forth veins through the killas of that country^]", a coarse 

 argillaceous schist, which is converted into hornblende-schist 



* Rev. W. Conybeare, Geol. Trans., 1st series, vol.iii. p. 201. 

 f Ibid., p. 205. I Ibid. p. 213, and Playfair, Illust. of Hutt. Theory, 253. 

 Ibid., p. 206. || Syst. of Geol., vol. i.p.206. ^[ See diagram, No. 87. 



