LAVA 



43 



is associated with other chemical differences which have a similar 

 effect upon fusibility, the basic kinds having much more lime, 

 magnesia, and iron in them, which act as fluxes. 



The experiments of Barus on the fusibility of lavas, which he 

 divides into three groups, resulted as follows: (i) Certain lavas 

 fuse readily (2250 F.) ; these are of basic composition and are 



Fig. 11. — Lava flow on Vesuvius, showing slaggy and scoriaceous surface. 



made up of lime-soda felspars (see p. 16), 1 the augitic and allied 

 ferro-magnesian minerals (p. 20), and iron oxide, but rarely have 

 quartz (p. 15). (2) A second group is of medium fusibility (25 20 

 F.), and is made up of lime-soda felspars, augitic or hornblende 

 minerals, and frequently quartz. (3) The third series melt with 

 difficulty (2700 F.), and remain pasty at even 3100 F. These 



1 For the sake of convenience, the minerals are all considered together in Chap- 

 ter I, but the student will greatly facilitate his acquaintance with them by referring 

 to the description of every mineral that he finds mentioned in the text, and es- 

 pecially by examining specimens. 



