372 GEOLOGICAL APPEARANCES 



spathose iron-ore, in nodules of argillaceous iron-stone. On the 

 other hand, that gradation is no decisive mark of secretion, is proved 

 by instances of it on the sides of veins, of which the posterior forma- 

 tion is undoubted. Werner mentions examples of it*; and Mr Ja- 

 meson observes in his Geognosy, where treating of such veins, that 

 there are " cases in which the substance of the vein is intimately mix- 

 " ed with its walls f." A fact of this kind was met with in Corn- 

 wall in the year 1799, when Mr Playfair and myself were in that 

 country. A mine near Redruth, called Toll Carne, was then work- 

 ing in a vein containing tinstone. The walls were of granite, and 

 crystals of tinstone were found disseminated through the granite, for a 

 few inches from the sides of the vein. It was regarded by some ex- 

 perienced miners of that district, as quite a singular instance ; but 

 Werner mentions similar facts concerning tinstone £. 



An analogous example of gradation was described to me by Sir. 

 Humphry Davy, as having been seen by Mr Greenough and him- 

 self, on the shore of Loch Erne in Ireland. A projecting rock of con- 

 siderable height, so as to render it a conspicuous object from a dis- 

 tance, was found to consist of a large vein of trap, or dyke of whinstone, 

 traversing strata of limestone nearly horizontal. At the distance of 

 some yards from the vein, the limestone presented the usual characters 

 of that substance ; but, on approaching the vein, it grew more in- 

 durated, and to all appearance siliceous, and was highly so, where in 

 contact. The softer parts of the strata were much worn away, and 

 had left the harder parts on the sides of the vein, to form with it a pro- 

 jecting mass. 



At Beadnel Bay, on the coast of Northumberland, there is a whin- 

 stone dyke, cutting strata, some of which are of limestone, and exhi- 

 bit near the dyke a change of character very analogous to that just 



described. 



* See Theory of Veins, § 58, 72, 75. 

 f P. 232. See also p. 239. 

 X See Theory of Veins, § 72. 



