Ch. XXVI.] 
BY VOLCANIC DIKES. 
369 
distinctly confined to those portions of the rock affected by the 
dike*. Garnets have been observed, under very analogous 
circumstances, in High Teesdale, by Professor Sedgwick, 
where they also occur in shale and limestone, altered by a 
basaltic dike. This discovery is most interesting, because 
garnets often abound in mica-schist, and we see in the instances 
above cited, that they did not previously exist in the shale and 
limestone, and that they have evidently been produced by heat 
in rocks in which the marks of stratification have not been 
effaced. 
Stirling Castle. — To select another example : we find in the 
rock of Stirling Castle, a calcareous sandstone fractured and 
forcibly displaced by a mass of green-stone, 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. So also in Arthur's Seat and Salisbury Craig, 
near Edinburgh, a sandstone is seen to come in contact with 
greenstone, and to be converted into a jaspideous rock f , 
Antrim. — In the north of Ireland, in several parts of the 
county of Antrim, chalk, with flints, is traversed by basaltic 
dikes. The chalk is converted into granular marble near the 
basalt, the change sometimes extending eight or ten feet from 
the wall of the dike, being greatest at that point, and thence 
gradually decreasing till it becomes evanescent. ' The extreme 
effect,' says Dr. Berger, ' presents a dark brown crystalline 
limestone, the crystals running in flakes as large as those of 
coarse primitive limestone ; the next state is saccharine, then 
fine-grained and arenaceous ; a compact variety having a por- 
cellanous aspect, and a bluish-grey colour succeeds ; this, 
towards the outer edge, becomes yellowish-white, and insen- 
sibly graduates into the unaltered chalk. The flints in the 
altered chalk usually assume a grey yellowish colour J.' All 
* Trans, of Cambridge Phil. Soc, vol. i. p. 406. 
f Must, of Hutt. Theory, § 253 and 261. Dr. Macculloch, Geol. Trans., 1st 
series, vol. ii. p. 305. 
X Dr. Berger, Geol. Trans., 1st series, vol. iii. p. 172. 
Vol. III. 2 B 
