866 



THE WONDERS OF GEOLOGY. Lect. VIII. 



supposition that they have been formed by intense igneous 

 action, is not only probable, but is rendered almost certain, 

 by the experiments of M. Gaudin, who succeeded in produc- 

 ing fictitious rubies, which in every respect resemble the 

 natural gems. These were formed by submitting alumina, 

 with a small quantity of calcined chromate of potash, to the 

 influence of a powerful oxy-hydrogen blowpipe, by which 

 the materials were melted into a crystalline mass, that pre- 

 sented, when cooled, all the characteristics of the Ruby. 



Garnet, is a well-known precious stone, of a rich brown- 

 ish red colour, and is generally found in plutonic rocks ; 

 like the ruby, it has also been made artificially, by exposing 

 its constituents, silicates of alumina, lime, iron, &c, to 

 intense heat. This experiment throws light on the occur- 

 rence of garnets in shale, altered by contact with a dike of 

 granite or trap, though altogether wanting in every other 

 part of the rock ; a proof that these crystals have been 

 produced by the effect of heat on those parts of the sedi- 

 mentary deposits which were most exposed to the influence 

 of the erupted mass.* 



The production of such crystalline substances, though 

 effected by intense heat, probably depends on the action of 

 electro -chemical currents induced by the high temperature ; 

 since M. Becquerel and Mr. Cross have formed without heat, 

 from solutions, by long-continued galvanic action, crystals 

 of quartz, arragonite, carbonates of lime, &c. which the 

 resources of chemistry had failed to yield. f 



32. Metalliferous veins. — In my description of the 

 fissures observable in consolidated strata, I mentioned that 



* Mr. Lyell. 



f " Light, Heat, Electricity, Magnetism, Motion, and Chemical- 

 affinity, are all convertible material affections ; assuming either as the 

 cause, one of the others will be the effect. These forces in their 

 varied natural action upon the surface of our planet, are continually 

 altering the nature of its external crust." — Lecture on the Progress o 

 Physical Science, by W. JR. Grove, Esq. F.B.S. 



