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 lllust. of Hutt. Theory, $ 253 and 261. Dr. Macculloch, Geol. Trans., 1st 

 series, vol. ii. p. 305. 



I Dr. Berger, Geol. Trans., 1st series, vol. iii. p. 172. 



VOL. III. 2 B 



