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Sect. VIIL] 



MINERALOGY, 



41 



remain, if not again covered in consequence of any of 

 those changes which may have taken place in the mineral 

 vein, and by which new mineral matter may be thrown 

 down, being of com^se liable, like any other of the pre- 

 existing minerals, to be covered up by deposits of this 

 kind. As might be expected, the minerals thus, as it 

 were, cast in a mould, vary conblderably ; silica, often as 

 chalcedony, being in some districts frequent 



When we find the hollows that have been left in rocks 

 by the disappearance of the original substances of shells, 

 and other organic remains which have been entombed in 

 them, filled by various mineral substances, some totally 

 different from the original matter of the organic remain, 

 such as silica, and the sulphurets of iron, copper, and lead 



pseudomorph 





minerals in the body of a rock itself. In spring waters^ 

 those clearly derived from rain percolating through rocks, 

 and thrown out on the sides of hills by some bed called 

 impervious, a term which should be only regarded as com- 

 parative, we find abundant evidence of the chemical solu- 

 tion of some parts of the beds which the water has passed 



through. 



In all kinds of 



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in volcanic countries rnay be regarded as w^ater formed by 

 the condensation of steam, we still find the same thing, so 

 that we are prepared for the filling up of hollows and 

 cavities, no matter how formed, by matter brought in 

 solution into them, and partly or wholly left there, ac- 

 cording to circumstances. Of this kind of filling up, the 

 vesicles and gas, vapour, or air-cavities of igneous rocks 

 of different geological ages afford us excellent examples, 

 more particularly when it is seen, as it often can be, that 



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