CHAP. XII. 



Chiloe. 



395 



ruginous, yet fusible, nature ; they are as round as 

 cannon-balls, and vary from two or three inches to two 

 feet in diameter ; their insides generally consist either 

 of fine, scarcely coherent volcanic sand, 1 or of an argil- 

 laceous tuff; in this latter case, the external crust was 

 quite thin and hard. Some of the spherical balls were 

 encircled, in the line of their equators, by a necklace- 

 like row of smaller concretions. Again there were other 

 concretions, irregularly formed, and composed of a hard, 

 compact, ash-coloured stone, with an almost porcelain- 

 ous fracture, adhesive to the tongue, and without any 

 calcareous matter. These beds are, also, interlaced by 

 many veins, containing gypsum, ferruginous matter, 

 calcareous spar, and agate. It was here seen with re- 

 markable distinctness, how intimately concretionary 

 action and the production of fissures and veins are re- 

 lated together. The following diagram is an accurate 



No. 36. 



Ground plan showing the relation between veins and concretionary zones 

 in a mass of tuff. 



representation of a horizontal space of tuff, about four 

 The frequent tendency in iron to form hollow concretions or shell 

 18 



