Ch. XIV ] OF THE STRATIFIED ROCKS. 305 



I 



contact ; where this is the case, the change sometimes extends 

 eight or ten feet from the wall of the dike, being at that point 

 greatest, and thence gradually decreasing, till it becomes eva- 

 nescent. The extreme effect 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- 

 celainous aspect, and a bluish grey colour, succeeds : this, to- 

 wards the outer edge, becomes yellowish white, and insensibly 

 graduates into the unaltered chalk. The flints in the altered 

 chalk usually assume a grey yellowish colour; the altered 

 chalk is highly phosphorescent when subjected to heat." * 



We must not quit the subject of the metamorphosis of 

 limestone by the contact of trap, without noticing the crys- 

 talline magnesian limestones which are supposed to have been 

 originally common limestone converted into dolomite by 

 porphyry and similar igneous rocks. Many objections might 

 be urged against the possibility of the introduction of mag- 

 nesia into this rock ; but the difficulties of the case have been 

 so fairly stated and discussed by De la Beche f, that we need 

 not insist on the imperfect state of this hypothesis. There is, 

 however, one point which may be alluded to, because it is 

 directly opposed to the nature of the supposed altered chalk 

 of Ireland; viz., the occurrence of organic remains in dolo- 

 mite, which, in the other case, are said to have been obliter- 

 ated by the fusion requisite to impart a crystalline texture to 

 the earthy carbonate. 



We have been thus particular in detailing these examples 

 at some length, not only because the original works may not 

 be within the reach of all our readers, but also because it is 

 important to bring all the minutiae of these cases into one 

 point of view. 



It is, therefore, by these, and numerous phenomena of the 

 same nature, clearly established that the stratified rocks, 



* Geol. Trans., vol. lit. p. 172. f Geol. Manual, vo., p. 474. etseq. 



X 



