93 



which alternate with volcanic depositions ? This 

 last hypothesis seems so much the less admissi- 

 ble, since, from the researches of Sir James Hall 

 on the influence of pressure in fusions, the exist- 

 ence of carbonic acid in substances contained in 

 basalt offers nothing surprising. Several lavas 

 of Vesuvius present similar phenomena, In 

 Lombardy, between Vicenza and Abano, where 

 the calcareous stone of the Jura * contains great 

 masses of basalt, I have seen this latter enter 

 into effervescence with the acids wherever it 

 touches the calcareous rock. 



We had not time to reach the summit of a hill, 

 that was very remarkable in having it's base 

 formed of banks of clay under strata of basalt, 

 like a mountain in Saxony -f, which is become 

 celebrated on account of the disputes of volca- 

 nean and neptunean geologists. These basalts 

 were covered with a mammaeform substance, 

 which I vainly sought on the Peak of Teneriffe, 

 and which is known by the name of volcanic 

 glass, glass of Muller or Hyalite ; it is the tran- 

 sition from the opal to the calcedony. We struck 

 off with difficulty some fine specimens, leaving 

 masses that were eight or ten inches square un- 

 touched. I never saw in Europe such fine hy- 

 alites as I found in the island of Graciosa, and 



* Jura-kalstein. 



f Scheibenbergen huegel. 



