220 Fresh Notes on the Crater Linne. 



twelfth more English feet, and would therefore be very nearly, 

 if not quite in a region of maximum volcanic force. 



The moon offers no 'positive evidences of having had its 

 surface modified by aqueous action, and if no such action has 

 occurred on an extensive scale, the appearances presented 

 may be those of a much more primitive condition than are 

 now found on our earth. The larger craters may date back to" 

 a period of igneous plasticity, and eruptive action may now be 

 on a much smaller scale, and also more local in character, than 

 it was in former ages when the surface was less solid, and the 

 heat below the surface more equally diffused and more intense. 



The matters erupted by terrestrial volcanoes are vaporous 

 and gaseous as well as solid, and water frequently occurs in 

 what are called " igneous rocks." Following Cottars division,* 

 such rocks are arranged in the two classes, volcanic a,xi& plutonic, 

 the former comprehending such as " have solidified at or near 

 the surface, and the latter those which have solidified at a con- 

 siderable depth in the interior of the earth." 



Taking the whole range of igneous rocks, Cotta gives the 

 following " extreme average values of their chemical consti- 

 tuents": — 



Silica 50—80 



Alumina 10 — 25 



Peroxide and protoxide of iron . 1 — 25 



Lime — 15 



Magnesia — 12 



Potash . 0—10 



Soda 0—7 



Water 0—5 



Another division of igneous rocks refers to their being 

 richer or poorer in silica, which behaves like an acid. Those 

 most rich in this constituent are called acidic, and those less 

 rich basic, and both kinds frequently contain small quantities 

 of water. 



When terrestrial eruptions occur, vapours of water, sulphur, 

 etc., and gases, are abundantly emitted. In the words of M. 

 St. Claire Deville, "lava never flows forth without bringing 

 with it immense quantities of gaseous matters and vapours. 

 These last may, indeed, appear singly, but everything indicates 

 that they escape from stony masses in fusion situated more 

 deeply ."t Water coming into contact with such masses must 

 be immediately converted into high pressure steam ; and 

 wherever large quantities of vaporized substances are emitted, 



* Cotta's Roclcs Classified and Described. — Longmans. 

 t Comples Rendiis, No. 26, I860. 



