LAUKENTIAN LIFE 



109 



the bowels of the eiirth which we in our experi- 

 ments cannot easily imitate or understand, namely, 

 the action of superheated water prevented by pres- 

 sure from escaping as steam, and permeating the 

 whole substance of deposits, which are thus baked 

 at a high temperature in presence of water, instead 

 of being exposed to mere dry heat, as in our kilns 

 and furnaces. The study of the partial changes 

 which have passed on later sediments where in 

 contact with volcanic masses once intensely heated, 

 enables us to understand the greater and more ex- 

 tensive metamorphism of the oldest rocks. Thus a 

 mere mud becomes glorified by metamorphic cry- 

 stallization into a micaceous schist. We have taken 

 ordinary clay as an example ; but under the same 

 processes sand has been converted into a compact 

 quartzite, ordinary limestone into crystalline marble, 

 clay-ironstone into magnetic iron ore, coal into 

 graphite, and lavas or volcanic ashes into hard 

 crystalline granites, gneisses, or pyroxene rocks or 

 hornblendic schists, according to their original com- 

 position. There may exist portions of these old 

 rocks which ha-^e been exempt from such alteration, 

 but hitherto wj have not been able to find them, 

 and they are probably under the ocean bed, or 



