VOLCANIC TITEFS 



123 



dust and fragments of basaltic lava, with pieces of a pale yellow, 

 green, reddish, or brownish glass called palagonite. The general 

 name of volcanic mud is given to the finely comminuted volcanic 

 material which in a more or less pasty or liquid condition is 

 thrown from volcanic vents during the incipient stages of 

 eruption. 



The tuffs are as a rule more or less distinctly stratified and of 

 very uneven texture. They are found associated with volcanic 

 rocks of all ages, and are at times so highly metamorphosed as to 

 render the original nature of some doubt. Certain English 

 authorities have contended that a part of the so-called argillites 

 and fire clays were of finely comminuted volcanic materials. 



The composition of the tuffs naturally varies with that of the 

 character of the lava from which they were derived. Being often 

 porous and readily permeated by water or rootlets, they undergo 

 decomposition, forming soils the character of which is dependent 

 to some extent upon their lithological nature. The following 

 table shows the varying composition of rocks of this class : 



Chemical Composition oi' Yolganic Tuffs 



Kinds and Localities 



o 



i 





--> 



OO 



1 



I 



e3 



IS 



1- 



o 



w 



1 



o 



% 



o 



w 



i 



fo 



00 





fo 



fo 



% 



fo 



fo 



fo 



Pozzuolana, Kaples, 





















Italy .... 



59.144 



21.28 



4.76 



1.90 



.... 



4.37 



6.23 



. • • 



97.68 



Tuff, Crater of Monte 





















Nuova, Italy . . 



66.31 



15.23 



7.11 



1.74 



1.36 



6.54 



4 84 



6.12 



99.25 



Trass, Andernach, 

 Prussia. . . . 













V < 







54.00 



16.50 



6.10 



4.00 



0.70 



10.00 



7.00 



98.20 



Tuff, Lacter See, 



















Prussia. • . . 



60.49 



19.95 



9.37 



3.12 



1.43 



3.40 



1.33 



99.09 



(2) EOCKS COMPOSED MAIOT^T OF DEBEIS PROM PLANT AND 



ANIMAL LIFE 



(1) The Siliceous Group: Diatomaceous Earth. — This is a 

 fine white or gray pulverulent rock, composed mainly of the 

 minute shells, or tests, of diatoms, and often so soft and friable 

 as to crumble readily between the thumb and fingers. It occurs 



