'236 Notice of Active and Extinct Volcanos. 



question, all the science which was necessary to his underta- 

 king — with a spirit of careful and accurate examination, and 

 a capacity for generalization, which enabled him to make the 

 best use of what he saw. His observations were made, prin- 

 cipally from 1819 to 1823. The following are the leading 

 divisions of his work. 



1 . The extinct volcanos of France and Germany. 



2. The volcanic districts of Hungary, Italy, Sicily and the 

 Lipari islands, including the active as well as dormant and 

 extinct volcanos of those countries — all the important points 

 of which, he examined in person. 



3. From the best authors, he describes the volcanos of oth- 

 er parts of the world, including the islands. 



4. The fourth division or lecture is devoted to general in- 

 ferences respecting volcanic phenomena. 



In his introduction, Professor Daubeny states the distinction 

 between active and extinct volcanos — the former including 

 all those which have been eruptive at any time since the ex- 

 istence of authentic records — the latter those that have, with- 

 in the same limits of time, exhibited no signs of activity, al- 

 though incontestably of the same origin. 



Thus, although a mountain should not exhibit a crater and 

 the usual figure and stratification of a volcano, — if its mate- 

 rials have " a vitreous aspect and fracture together with a cel- 

 lular structure — cells generally empty and elongated in the 

 same direction and if they have a glazed internal appearance" 

 there need be no hesitation in pronouncing that the materials 

 are of volcanic origin. 



All volcanic rocks may be included under 



1. Trachyte, 



2. Basaltic lava. 



1. Trachyte— (from the Greek tpaxvs) so denominated from 

 the harsh earthy feel which it often possesses — " is essentially 

 composed of crystals of glassy felspar, often cracked, which 

 are imbedded in a basis generally considered as being itself a 

 modification of compact felspar. To this are sometimes su- 

 peradded crystals of hornblende, mica, iron pyrites, specular 

 iron and more rarely augite and magnetic or titaniferous 

 iron." 



2. Basaltic lava — " appears to be some modification of ba- 

 salt — consisting essentially of augite, felspar and titaniferous 

 or magnetic iron generally accompanied with olivine and 

 sometimes with hornblende." 



