STJMMAEY OP THE CONTENTS OF VOL. II. 335 



remarkable of which we possess any certain knowledge since the 

 death of the elder Pliny 228235 



Difference between volcanos with permanent craters ; and the 

 phenomena (very rarely observed within historic times) in which 

 trachytic mountains open suddenly, emit lava and ashes, and 

 reclose again perhaps for ever. The latter class of phenomena 

 are particularly instructive to the geologist, because they recall the 

 earliest revolutions of the oscillating, upheaved, and fissured 

 surface of the globe. They led, in classical antiquity, to the view 

 of the Pyriphlegethon. Yolcanos are intermitting earth springs, 

 indicating a communication (permanent or transient) between the 

 interior and the exterior of our planet ; they are the result of a reac- 

 tion of the still fluid interior against the crust of the earth ; it is 

 therefore needless to ask what chemical substance burns, or 

 supplies materials for combustion, in volcanos . . 235 238 



The primitive cause of subterranean heat is, as in all planets, the process 

 of formation itself, i. e. the forming of the aggregating mass from a cos - 

 mical gaseous fluid. Power and influence of the radiationof heat from 

 numerous open fissures and unfilled veins in the ancient world. 

 Climate (or atmospheric temperature) at that period very indepen- 

 dent of the geographical latitude, or of the position of the planet in 

 respect to the central body, the sun. Organic forms of the present 

 tropical world buried in the icy regions of the north 238 241 



Scientific Elucidations and Additions}*. 243 to p. 248. 



Barometric measurements of Vesuvius. Comparison of the height 

 of different points of the crater of Vesuvius . . 243 247 



Increase of temperature with depth, 1 Reaumur for every 113 

 Parisian feet, or 1 of Fahrenheit for every 5 3 '5 English feet. 

 Temperature of the Artesian well at Oeynhausen's Bad (New 



