92 



or that the porous rock lay on compact strata, as 

 happens in the currents of lava of iEtna and 

 Vesuvius. The marl *, which alternates more 

 than a hundred times with the basalts, is yellow- 

 ish, friable by decomposition, very coherent in the 

 inside, and often divided into irregular prisms, 

 analogous to the basaltic prisms. The sun dis- 

 colours their surface, as it whitens several schists., 

 by reviving a hydrocarburetted principle, which 

 appears to be combined with the earth. The marl 

 of Graciosa contains a great quantity of chalk, 

 and strongly effervesces with nitric acid, even on 

 points where it is found in contact with the ba- 

 salt. This fact is so much more remarkable, as 

 this substance does not fill the fissures of the 

 rock, but it's strata are parallel to those of the 

 basalt ; whence we may conclude, that both fos- 

 sils are of the same formation, and have a com- 

 mon origin. The phenomenon of a basaltic rock 

 containing masses of indurated marl split into 

 small columns, is also found in the Mittelgebirge, 

 in Bohemia. Visiting those countries in 1792, 

 in company with Mr. Freiesleben we even re- 

 cognized in the marl of the Stiefelberg the imprint 

 of a plant nearly resembling the cerastium, or the 

 alsine. Are these strata, contained in the trap- 

 pean mountains, owing to muddy irruptions? or 

 must we consider them as sediments of water, 



* Mergel. 



i Bergmsennisches Journal, 1792, p. 215. 



