NOME IV. TUFO. 425 



near Frankfort, Cologne, Pleith, &c. and there 

 called tuffstein. 



" Its colour is grey, brown, or yellowish. 



" Its surface rough and porous. 



" Its lustre and transparency, 0. 



" Its fracture, commonly earthy, rarely lamel- 

 lar ; it contains fragments resembling pumice 

 (though not real pumice, Voigt Fulda, 221) ; also 

 fragments of argillite and basaltin (siderite) ; often 

 branches of trees half cleared, and impressions of 

 leaves, 2 Nose, 182. Mica, iron ore, and other 

 heterogeneities, are more frequent in it than in 

 puzzolana, 3 Bergm. 196. 



" Its hardness from 5 to 7. 



" Feels dry and harsh. Scarcely effervesces 

 with acids. 



" It is not diffusible in cold water ; but in hot 

 it gives an earthy smell, and deposites a finer 

 earth. 



" It melts into a greyish brown slag. 



" It is found in valleys, some feet under the 

 surface, to which no streams of water have had 

 access. Sometimes in columnar masses of a grey, 

 or Isabella yellow colour, some round and some 

 quadrangular, standing close to each other, and 

 forming internally one common mass. 3 Berl. 

 Beob. 199. 



" According to Mr. Bergman, it consists of 



