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'tbis 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 -f- ; 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 coars 
argillaceous schist, which is converted into hornblende-schist 
* Rev. W. Conybeare, Geol. Trans., 1st series, vol.iii. p. 201. 
f Ibid., p. 205. % Ibid. p. 213, and Playfair, Illust. of Hutt. Theory, § 253. 
§ Ibid., p. 206. || Syst. of Geol./vol. i.p.206. f See diagram, No. 87. 
