122 AQUEOUS BOOKS 



dwelt upon here It may be stated, however, that owing to the 

 softness and ready solubility o£ their materials limestones do not, 

 on breaking down, except nnder rare instances, give rise to ex- 

 tensive beds of arenaceous rocks, as do the siliceous varieties. 

 One of the best known rocks of this group is the breccia marble 

 near Point of Rocks in Maryland, which has been used for deco- 

 rative purposes in the Capitol building at Washington. 



(4) The Volcanic Group: Tuffs — Under this head are in- 

 cluded a great variety of fragmental rocks, composed of the 

 more or less finely comminuted materials ejected from vol- 

 canoes as ashes, dust, sand, and lapilli. These occur, in many 

 instances, interbedded with lava flows of the same lithological 

 nature, and are a product of the same periods of volcanic 

 activity, the eruption of molten lava being interrupted by 

 intervals of explosive action, during which only fragmental 

 material was ejected. To such materials the name p^/roclastic 

 (Greek T,op6<s^ fire) is appropriately given. 



The lithological character of the materials varies greatly, and 

 only very general names are given them in the majority of cases. 

 The name tiijf or tiiifa is given to the entire group formed as 

 above, and also by some authorities to fragmental rocks resulting 

 from the breaking down and reconsolidation of older volcanic 

 lavas. It would seem advisable to designate these last, as has F. 

 Lowinson-Lessing,^ as pseudotuffs or tuffoids. 



The names volcanic ashes, sand, and dust are applied to 

 the finer of these volcanic materials in an unconsolidated state 

 and lapilli or rapilli to the coarser fragments. 



The dusts and sands are not infrequently composed of 

 minute shreds of volcanic glass, which were blown from the 

 volcanic vents and carried unknown distances, to be ultimately 

 deposited as stratified beds in comparatively shallow water. 

 Such are described more in detail under the head of ^olian 

 rocks (p. 133). The term trass is used to designate a compact 

 or earthy fragmental rock composed of pumice dust, in which 

 are embedded fragments of trachytic and basaltic rocks, car- 

 bonized wood, etc., and which occupies some of the valleys of 

 the Eifel. Peperino is a tufaceous rock composed of fragments 

 of basalt, leucite lava, and limestone, with abundant crystals 

 of augite, mica, leucite, and magnetite. It occurs among the 

 Alban Hills, near Eome, Italy. Palagonite tuff is composed of 



^ Tschermaks, Min. n. Petrog. Mittheilungen, Vol. IX, 1889, p. 530. 



