SALMON — OX THE F0EMATI02^ OF OEE-YEINS. 



391 



III. Baiyte and Fluor-spar with Galena, and some otlier ores, dis- 

 posed in very regular layers. Predominates nortli of Freiberg. 



The order in which these combinations are given is also, at the same 

 time, that of their relative age, and from this we can easily imder- 

 stand how the oldest combination is characterized by quartz and 

 fragTQents of the neighbouring rocks, while the newest is marked by 

 a particularly regular layer-like form. 



According to our theory, the Freiberger ore-veins are due to the 

 porphyry-eruptions. The eruptive force opened fissures fi^om time 

 to time, during a long period; and through these fissm^es water, 

 perhaps sea-water, penetrating to a great depth, found in the still- 

 heated deeply-lying porphyry-masses (which at Freiberg only pene- 

 trate to the surface in isolated dykes) those earths and metals which 

 are soluble under a high pressm-e at a high teniperatui'e and ^sith the 

 co-operation of alcalies ; and which it (the water) then subsequently 

 deposited in its circulation thi'ough the upper and colder regions of 

 the fissures. Of the earths, silicic acid, when dissolved in great 

 quantities, crystallized again at a very high temperatm^e. For this 

 reason, it fills principally the oldest fissures, or forms the oldest por- 

 tions in newer ones, or in those which, occupying a medium position 

 in the whole vein-region, preserved longest a very high temperature. 

 The other minerals followed it more or less periodically, according to 

 their capacities for reduction or solidification, which certainly cannot 

 as yet be specially authenticated, although G. Bischoff has already 

 done much with reference to it. The depositions in the fissures en- 

 sued at first very energetically and under continued disturbances, 

 whence arises their breccia-fonn and the massiveness of their con- 

 tents ; while later, with a decreasing temperature and less dis- 

 turbances, and with slower deposition, the layer-like form gradually 

 made its appearance. Yapom^- and gas-streams may have occurred 

 later, in place of the slow continuous circulation of water, and accu- 

 mulated in drusy cavities or more recent sublimation -clefts, or, by a 

 change in existing contents, have introduced metamorphism and 

 translocation. But as, according to this theory, the deep-lying 

 porphyry-masses are the original bearers of the contents of the Frei- 

 berg ore-veins, it should incite us to a further investigation, which 

 might besides be important in a practical point of view. 



