51,0 DOMAIN XII. VOLCANIC. 



fragments the size of an olive, and sometimes of 

 a nut, adhering by the points of contact, the 

 matter which unites them not. being distinguish- 

 able. This tufo is exceeding light, but not of a 

 strong consistence. 



<c Q. Tufo, whose base is a pumice reduced 

 into so fine a powder that it has the appearance 

 of an argillaceous substance : this unites a num- 

 ber of very small grains of pumice, dryer, harsher 

 to the feel, and much less altered, and very dis- 

 tinct pieces of porous lava, although partly dis- 

 coloured. This tufo forms one of the varieties 

 of trass of Pley t, in the environs of Andernach. 

 What I have said of it in a distinct memoir may 

 be consulted, in which I have described the se- 

 veral considerable quarries of these tarrasses, 

 which are wrought to be converted into cement. 

 See Annales da Museum, vol. i. p. 15. 



cc 3. Tufo, formed of a mixture of pumice in 

 powder or in grains, angular fragments of black 

 compact basaltic lava, and small scaly fragments 

 of a grey schistus, rather shining, not volcanic, 

 which has been cast up with the other sub- 

 stances. It is in this variety of tufo, which has 

 much more solidity than the preceding, and 

 which has formed beds and masses more than 

 fifty feet thick, that there are sometimes found 

 cylindrical pieces of real charcoal, as sound and 



