868 



THE WONDERS OF GEOLOGY. Lect. VIII. 



great weight to the hypothesis which explains the filling up 

 of metallic veins by electrical agency. M. Becquerel re- 

 marks, that when a vein is filled either wholly or partially, 

 the transfusion of water from the surrounding rocks would 

 bring electric forces into play, and give rise to decomposi- 

 tions and new combinations of mineral matter. The sepa- 

 ration of pure metal from solutions of metallic salts, by 

 galvanic action, — a process familiar to every one, under the 

 name of the electrotype, — and from the ore by a modification 

 of the same force, exemplifies the nature of those changes 

 by which native gold, silver, copper, &c. may be produced 

 in the interior of the earth.* 



There appear to be certain associations of metallic sub- 

 stances in the veins ; as, for instance, iron and copper, lead 

 and zinc, tin and copper ; and those ores which are combined 

 with a similar base, as sulphurets, carbonates, phosphates, 

 arseniates, &c. are commonly found together, j The follow- 

 ing list shows the geological distribution of a few of the 

 chief metals. 



Tin— generally occurs in quartz veins traversing granite and schist. 

 It has not been discovered in a native state, but is commonly 

 found as an oxide, and rarely as a sulphuret. The ores of this 

 metal are of great hardness and specific gravity, and are termed 

 tin-stone. Wood-tin, so called from its fibrous structure, and 

 stream-tin, are found in the beds of streams and rivulets : they 

 are the alluvial detritus of tin-veins that existed in rocks now 

 destroyed. The stanniferous gravel of Cornwall is the debris of 

 pre-existing rocks traversed by tin-veins, and has been formed 

 in the same manner as the auriferous alluvia of Eussia. The 

 mines of Cornwall are the most productive in Europe, and have 

 been worked from the remotest historical periods. The Tyrians, 

 as early as the time of Moses, imported tin from that district. 



* M. Becquerel has succeeded, by the permanent action of electrical 

 currents only, in separating the metals of silver, lead, and copper, from 

 their ores. The electro-chemical apparatus employed consisted simply 

 of iron, a concentrated solution of sea-salt, and the ore of the metal 

 properly prepared. See notes to Dr. Buckland's Bridgewater Essay. 



f Professor Phillips. 



