296 



FORMATION OF METALLIC ORES. 



When a portion of the wall was protected from access to the at- 

 mosphere by glass, which projected a little distance from the surface, 

 tlie formation of nitre went on for a certain time, and then ceased. 

 The saline crystals were better defined, and longer, than on the 

 other parts of the wall. When the wall was coated with paint, crys- 

 tals of nitre were even formed on the paint. The formation of car- 

 bonate of lead on the walls of the mine at Wolfclough, may be analo- 

 gous to the formation of nitre ; and in both instances, the surface of 

 the wall and of the atmosphere, may perhaps be considered as two 

 galvanic plates in action, decomposing and recompounding the ele- 

 ments of metallic or saline matter from the atmosphere, or the gase- 

 ous fluids with which it is intermixed. The base of nitre (potas- 

 sium) is known to be a metal ; and could we seize nature in the act 

 of producing a fixed alkali from more simple elements, we might 

 compel her to reveal the process by which she prepares her metallic 

 treasures in the deep recesses of the earth. Nor can the discovery 

 be very remote ; for we are already acquainted with the composition 

 of the volatile alkali, and are thereby enabled successfully to imitate 

 nature in its formation. 



When the matrix, or the substance which principally fills veins, 

 is a soft unctuous clay, masses and particles of ore are often dissem- 

 inated through it, varying in size from a pea to that of a large gourd, 

 and they are sometimes even of many ton's weight. Masses of vein- 

 stone are also imbedded in the same manner; and it is observed 

 that the masses both of ore and veinstone are of no determinate 

 shape, and have generally the appearance of being corroded. Are 

 we to conclude, in such instances, that the hard minerals and metallic 

 ores, have been formed in the substance of the clay by some peculiar 

 elective affinity, or that they once occupied the cavity of the vein, 

 and have been all subsequently decomposed, except the remaining 

 detached masses ? I should be more inclined to adopt the former 

 opinion ; but it must be allowed, that there are inexplicable instan- 

 ces of the disappearance of minerals which formerly existed in veins. 



The formation of one mineral upon the crystals of another, and 

 the disappearance of the crystal which has served as the mould, is 

 indeed a common phenomenon in many English mines. I have be- 

 fore me a mass of rock crystal from Durham, formed on cubic fluor 

 spar ; but the crystals of the latter have entirely disappeared, leav- 

 ing nothing but the impression of their form. In the mines of Der- 

 byshire, incrustations of calamine are formed on calcareous crystals, 

 taking the shape of the dog-tooth spar ; but in these false crystals, 

 no trace of the interior crystal is left. Certain local causes also ap- 

 pear to influence the crystallization of minerals in different districts, 

 and to dispose them to take peculiar secondary forms, which may be 

 considered as appropriate to the minerals of that district. The py- 

 ramidal crystallization of carbonate of lime, called the dog-tooth 

 spar ( chaux carbonatee metastatique of Haiiy,) is abundant in some 



