Mr. Lyell's Address. 395 



by one who has acquired much practical knowledge as a miner, and 

 who is well versed in chemistry and mineralogy*. 



Werner, when he pubUshed his justly celebrated Essay on Mine- 

 ral Veins, had come to the conclusion that the same rent, after being 

 wholly or partially filled, has sometimes been reopened; and M. 

 Fournet has endeavoured more fully to explain the successive dila- 

 tation of the same veins at distinct periods. He has given examples 

 in mines worked under his direction in Auvergne, in which the sul- 

 phurets of iron, copper, lead, and zinc, besides quartz, barytes, and 

 other minerals, seem evidently to have been introduced at different 

 periods by chemic d action accompanied by new fractures and dis- 

 locations of the rocks, and the widening of preexisting fissuresf . 



You will find in M. Fournet's treatise a copious analysis of a great 

 variety of books on mining, besides a detail of facts which have 

 fallen under his own observation. He has described first those 

 veins which are decidedly connected with rents produced in rocks 

 by mechanical movements, and which are supposed to have been 

 chiefly filled from below by sublimation, more or less obviously con- 

 nected with volcanic action. He afterwards passes on to the con- 

 sideration of those masses which have been called stockwerks by 

 the Germans, which are imagined by some to have their origin in 

 the contraction of granite, porphyry, and other rocks as they cooled, 

 numerous rents being then formed, in which metallic particles were 

 concentrated. In treating the subject in this order the author ap- 

 pears to me to have followed the most philosophical course, begin- 

 ning with cases of undoubted rents of mechanical origin filled with 

 minerals and metals introduced by sublimation, and then carry- 

 ing with him as far as possible the light derived from these 

 sources to dissipate a part of the obscurity in which all theories re- 

 specting the nature of Plutonic rocks and their minerals must, I fear, 

 be for ever involved. Much will still remain unexplained; but 

 those who proceed in an opposite direction often throw doubt and 

 confusion upon the simplest phaenomena, as has sometimes happened 

 in an analogous case, when geologists have begun with the exami- 

 nation of granite and granite veins, and have then endeavoured to 

 apply the ideas derived from this study to the trap rocks and vol- 

 canic dykes. 



Among the most interesting conclusions deduced by M. Fournet 

 from his examination of the mining districts of Europe, I may men- 

 tion the modern periods at which the precious metals apjjear to have 

 entered into some veins: thus, to select a single example, some veins 

 of silver of Joachimsthal in Bohemia are proved to have originated 

 iu the tertiary periodj. 



FOREIGN GEOLOGY. 



Among the researches into the geology of foreign countries in 

 which our members have been recently engaged, J have great 



* Etudes sur les D6pots Metalliferes, par M. I. Fouruet. 

 .f See " Etudes," &r. Section 3. 

 J See " Etudes," &c. Section 2. 

 3E2 



