Park. — Metasomatism in Formation of Ore-deposits. 35 



of great intensity. It has been proved that when a por- 

 tion of dissolved substance is deposited from a solution at any 

 point the osmotic balance is disturbed, and immediately more 

 dissolved matter travels to that point, in accordance with the 

 well-established laws of osmotic diffusion, thereby providing 

 new matter to augment the growing mass of ore. Osmotic 

 pressure is the chemical principle which compels solutions to 

 maintain an equal state of concentration throughout their 

 whole mass ; and since it is always called into being when 

 precipitation commences, its operation as an agency in vein- 

 filling must not be overlooked. 



Metasomatism is a process of lode-formation, and does 

 not concern itself with the source or origin of the dissolved 

 matter contained in the solutions. It is almost certain that 

 metasomatic processes, to a greater or less degree, were active 

 agencies in the formation and filling of the majority of pyritic 

 ore-bodies. 



Veins in which the mineral contents are arranged in 

 symmetrical bands or crustifications can only be satisfactorily 

 explained by supposing that the vein-matter was deposited in 

 open channels, beginning with a crust on each wall, followed 

 by subsequent crusts until the channel became closed or the 

 solution exhausted. It is not assumed that the vein fissure 

 remained open its full width during the whole period of de- 

 position of the vein-matter. It is more reasonable to suppose 

 that the fissure gradually opened as the process of deposition 

 proceeded, the newly formed matter affording the necessary 

 support to the walls. The forces which initiated the fracture, 

 if still in existence, would doubtless tend to reopen and widen 

 the fissure from time to time. 



Waldemar Lindgren's classic paper on " Metasomatic 

 Processes in Fissure-veins " :;: represents a great advance in 

 the scientific investigation of vein-formation. The author has 

 followed Stelzner's methods of microscopic chemical research 

 with conspicuous success, in a field hitherto much neglected. 

 His work further shows that a clear understanding of the 

 genesis of a vein can only be obtained by a minute study of 

 the rocks contiguous to the ore-body. The metasomatism he 

 describes is clearly not correlative with the metasomatic 

 replacement defined by Emmons, but merely mineral pseudo- 

 morphism on a large scale. He defines his standpoint by 

 repeating and adopting Becker's statement! that " the theo.-y 

 of the substitution of ore for rock is to be accepted only 

 when there is definite evidence of pseudomorphic molecular 

 replacement." He mentions that quartz is found replacing 



* Lindgren, Trans. Amer. Inat. Min. Eng., vol. xxx, 1900, p. 578. 

 t Becker, Discussion, "Genesis of Ore-deposits," 1901, p. 204. 



