VOLCANIC ROCKS. 605 



It is barely sufficient here to notice, that certain 

 burnt clays, known by the name of porcelain- 

 jasper, have been considered as pseudo- volcanic 

 substances. They are, in every sense of the 

 word, artificial, and no more deserving of a place 

 than other products of common fire. They 

 occur in the vicinity of coal beds which have 

 been casually ignited. 



