302 ON THE IGNEOUS METAMORPHOSIS [Ch. XIV. 



great hardness and a jaspideous aspect, similar to that sandstone 

 which is in contact with the greenstone of Stirling Castle. 



" Considering, therefore, the analogy of these two sand- 

 stones, we may fairly conclude," says Dr. Macculloch, " that 

 they have, in these instances, been altered from their original 

 texture, in consequence of the proximity of the trap-rock : 

 and we have an equal right to conclude that the same influ- 

 ence has also converted the shale into Lydian-stone. This, 

 in fact, is the position of every specimen of this stone which 

 I have seen in Scotland. In Conachan, in Raasa, in Shiant, 

 at Talisker, it forms beds in contact with and involved in 

 trap, which, from their connection and position, appear to 

 have been common clay-slate."* 



But one of the most important examples which has hitherto 

 been adduced on this subject, is the modification of strata 

 next a basalt dike, near Plas Newydd, in Anglesea, described 

 by Professor Henslowe in the Cambridge Philosophical 

 Transactions. " The dike (we quote from Lyell) is 134 feet 

 wide, and cuts perpendicularly through strata of shale and 

 argillaceous limestone, which are altered to the extent of 

 thirty feet from the dike. The shale, as it approaches the 

 basalt, becomes gradually more compact, and is most in- 

 durated nearest the junction, where it loses part of its schis- 

 tose structure, but the separation into parallel layers is still 

 discernible. In several places the shale is converted into 

 hard porcelainous jasper : in the hardest parts of which the 

 fossil shells, principally Productse, are nearly obliterated ; yet 

 even here their impressions may be frequently traced. The 

 argillaceous limestone undergoes analogous mutations, losing 

 its earthy texture as it approaches the dike, and becoming 

 granular and crystalline. But the most extraordinary phe- 

 nomenon is the appearance in the shale of numerous crystals 

 of analcime and garnet, which are distinctly confined to those 

 portions of the rock affected by the dike."f 



* Geol. Trans, vol. iii. p. 99. 



t Principles of Geology, vol. iii. p. 368. 



