320 PROCEEDINGS OF THE ACADEMY OP [1884. 



which is observed along the River Alignon in the Ardeche, was 

 first critically discussed by the late Poulett Scrope, who investi- 

 gated its causes midst the volcanic debris of Central France, and 

 clearly determined that it was the result of irregular convection 

 and radiation of heat, and consequent irregular solidification. 

 The deep la^^ers, where the loss of heat was effected slowly 

 through conduction with the underlying rock, produced stout 

 vertical columns; the more superficial layers, where radiation 

 was most active, frequently produced horizontal columns, while 

 between the two were found columns occupying all the interme- 

 diate positions. 



