VOLCANIC ROCKS. 603 



this division. They are highly interesting in a 

 geological view, and deserve a careful examina- 

 tion. It must be remarked of the whole of these 

 lavas, that they present many modifications of 

 external form, arising from the manner in which 

 they have flowed, and resembling those found in 

 the slags of furnaces. 



SIXTH DIVISION. 



Ejected substances, more or less altered by 

 the fire. 



FIRST SUBDIVISION. 



Solid ; conglomerates. 



A. Conglomerates of various fragments of dif- 

 ferent rocks, with mica, augit, and other minerals. 



B. Conglomerates consisting chiefly of clay, and 

 having apparently been ejected in the state qf mud. 

 Tufo. 



a. Coarse tufaceous conglomerates. 



b. Fine and powdery tufa. 



The local varieties of these latter, such as 

 those of Herculaneum and Rome, are very in- 

 teresting; but they do not admit of enumeration 

 in this place. They often contain, augit, mica, 

 or other imbedded minerals, as well as the solid 

 lavas. 



