497 



upon chalk, as the equivalents of the lower Woolwich deposit, ob- 

 serving that the shells agree with those of the London clay. These 

 remarks seem to confirm the conclusion to which he had been pre- 

 viously led by the grand section at Alum Bay in the Isle of Wight, 

 namely, that the beds usually styled plastic and London clays be- 

 long to one zoological period. 



MINERAL VEINS. 



Your attention has been c died to the origin of mineral veins by 

 Mr. Fox, who has endeavoured to explain why so large a propor- 

 tion of the metalliferous veins in England and other parts of the 

 world should have an east and west direction. He supposes fis- 

 sures filled with water, containing sulphurets and muriates of cop- 

 per, tin, iron, and zinc in solution, through which currents of voltaic 

 electricity are transmitted. The metals separated from their sol- 

 vents by this action are deposited in the veins, and most abundantly 

 in veins running at right angles to the direction of the earth's mag- 

 netism ; for as the magnetic currents of the earth pass from north 

 to south, they cause those of electricity to move east and west, al- 

 though considerable deviations from this direction must be occa- 

 sioned in the course of geological epochs by variations in the mag- 

 netic meridian. 



Since Mr. Fox first ascertained the existence of electric currents 

 in some of the metalliferous veins in Cornwall*, Mr. Henwood has 

 made many experiments on the same subject, together with obser- 

 vations on the distribution of metallic and earthy minerals in veins. 

 He considers the results obtained by him to be in a great degree op- 

 posed to the theory of Mr. Foxf. 



Mr. Fox conceives the fissures in which metalliferous substances 

 occur, to have been at first small and narrow, and to have increased gra- 

 dually in their dimensions. This doctrine has also been propounded 

 in a work with which you are probably familiar, and from which I 

 have derived much instruction, I mean M. Fournet's Essay on Me- 

 talliferous Deposits. This Essay was originally included in the 3rd 



* Phil. Trans. 1830, p. 399. 



f See Mining Journal, Supplement 9. p. 34, December 1836, and Annals 

 of Electricity, No. 2. vol.i. on Electric Currents, &c. by W. T. Henwood 

 Esq. 



