256 GEOLOGICAL DESCRIPTION OF THE EARTHS CRUST. 



teach us how, in earlier terrestrial or geological epochs, 

 metalliferous or mineral veins may have been produced, 

 wherever the crust of our planet, then thinner, and fre- 

 quently rent by earthquakes, was fractured and fissured in 

 every direction by change of volume in cooling, presenting 

 communications with the interior, and a means of escape for 

 ascending vapours and sublimations of the metals and the 

 earths. The arrangement of the particles in layers parallel 

 with the bounding surfaces of veins, the regular repetition 

 of layers of the same materials on opposite parts, for example 

 on the walls, of veins, and the elongated drusy cavities 

 occupying the middle space, often furnish direct evidence of 

 the plutonic act of sublimation in metalliferous veins. As 

 theiveins or dykes which traverse rocks are more recent 

 than the rocks which are traversed by them, the relative 

 positions of the porphyry and of the argentiferous ores 

 in the mines of Saxony, which are the richest and most 

 important in Germany, teach us that they are at least more 

 recent than the remains of vegetation in the coal measures 

 and in the lower portions of the new red sandstone (Roth- 

 liegendes. 291 ) 



Our geological hypotheses of the formation of the crust 

 of the earth, and of the metamorphism of rocks, have 

 derived unexpected elucidation, from the comparison of 

 minerals elaborated by nature with the products of our 

 smelting furnaces, and from the endeavours to reproduce 

 the former artificially from their elements. ( 292 ) In all these ;' 

 operations, the same affinities which determine chemical 

 combinations are in action both in our laboratories and 

 in the bosom of the earth. Amongst the minerals 



