The Paragenetic Relations of Minerals. 101 



The artificial substances which most resemble rocks are un- 

 doubtedly the slags of smelting furnaces and glasses. Many 

 of the former are incorrectly regarded as un crystalline, and 

 those which are sub- or mono-silicates, although in large pieces 

 their fracture is conchoidal, almost always possess a granular 

 crystalline structure. The higher silicates are generally true 

 glasses. Both in the one and the other, vesicular cavities 

 occur very commonly, which are considered to have been 

 caused by the disengagement of gas during their formation. 



Many lavas, especially those of active volcanoes, are closely 

 analogous to these slags and glasses, and are quite as vesi- 

 cular as the crystalline slags of the Freiberg and other silver 

 works. Obsidian, a true natural glass, is almost always 

 vesicular. 



In the lavas belonging to more remote, although historic 

 periods, minerals are now and then found, whose chemical 

 nature does not admit of their being regarded as original 

 constituents. Thus, for instance, gypsum and vivianite, 

 Fe O 3 Po 5 8 H 0, which it cannot for a moment be doubted 

 are relatively of very recent formation. Crystals of gyp- 

 sum are likewise met with in the vesicles of the slags 

 from the Muldner Hiitten (Freiberg). Moreover, the greater 

 part of those rocks in which interesting associations of mi- 

 nerals in vesicular cavities are met with, are eruptive rocks. 

 It may therefore be fairly inferred that the formation of these 

 cavities is owing to a disengagement of gas. These vesicular 

 rocks, at the same time, almost always possess a porphyri- 

 tic structure ; but, what is still more remarkable, they are 

 seldom met with in the state in which it is probable they for- 

 merly existed. They are frequently disintegrated, the fracture 

 generally dull, they present no distinctive mineralogical cha- 

 racters, and may have been basalt, melaphyr, trachyte, diorite, 

 or perhaps lava, dolorite, &c. Amygdaloids likewise are very 

 rarely fresh rocks. Even furnace slags are altered when not 

 piled up in heaps, from which the meteoric water cau readily 

 run off. The originally sharp-cornered fragments break 

 down and cohere, forming in the course of time a compact 

 mass, which frequently does not resemble the original sub- 

 stance in any single character. The atmospheric influence 



