ON ITS EXTEEIOR. VOLCANOES. 421 



near the equator (Tolima, Purace, Pasto, Tuqueres, and 

 Cumbal), found, together with much aqueous vapour, 

 carbonic acid and sulphuretted hydrogen, but no mu- 

 riatic acid, no nitrogen, and no free hydrogen. ( 58 ) The 

 influence which the interior of our planet still exercises 

 on the chemical composition of the atmosphere, by 

 withdrawing these substances to return them again 

 under other forms, is no doubt a very inconsiderable 

 part of the chemical revolutions which, in primeval 

 times, the atmosphere must have undergone at the 

 breaking forth of great mountain masses from open fis- 

 sures. The conjectures which have been formed respect- 

 ing the probably very large portion of carbonic acid gas 

 in the ancient atmosphere, have been strengthened by 

 the comparison of the thickness of coal deposits with 

 the very thin carboniferous stratum (only seven lines in 

 thickness), which, in the temperate zone, according to 

 Chevandier's calculations, our thickest forests would 

 supply to the ground in 100 years. (- 1581 ) 



In the infancy of geology, prior to Dolomieu's saga- 

 cious conjectures, the source of volcanic activity was 

 not placed beneath the oldest rocks, which were then 

 universally held to be granite and gneiss. Resting on 

 some feeble analogies of inflammability, it was long 

 believed that the source of volcanic eruptions, and of 

 the emanations of gas occasioned by them, and lasting 

 for many centuries, was to be sought for in the combus- 

 tible materials contained in the newer upper silurian 

 sedimentary strata. More general knowledge of the 

 earth's surface, more profound and better directed geo- 

 logical researches, and the beneficial influence which 

 the great advances of modern chemistry have exercised 



