98 



LYELL'S ELEMENTS OF GEOLOGY. 



Trap Tuff Volcanic Tuff. 



of liquid matter, which often precedes the main current, or by 

 contact with water in or upon the damp soil. 



The more compact lavas are often porphyritic, but even the 

 scoriaceous part sometimes contains imperfect crystals, which 

 have been derived from some older rocks, in which the crystals 

 pre-existed, but were not melted, as being more infusible in their 

 nature. 



Although melted matter rising in a crater, and even that which 

 enters rents on the side of a crater, is called lava, yet this term 

 belongs more properly to that which has flowed either in the open 

 air or on the bed of a lake or sea. If the same fluid has not 

 reached the surface, but has been merely injected into fissures 

 below ground, it is called trap. 



There is every variety of composition in lavas ; some are 

 trachytic, as in the Peak of Teneriffe ; a great number are ba- 

 saltic, as in Vesuvius and Auvergne ; others are Andesitic, as 

 those of Chili ; some of the most modern in Vesuvius consist of 

 green augite, and many of those of Etna of augite and Labra- 

 dor-felspar.* 



Trap-tvff, volcanic-tuff. — Small angular fragments of the 

 scorice and pumice, above mentioned, and the dust of the same, 

 produced by volcanic explosions, form the tuffs which abound in 

 all regions of active volcanos, where showers of these materials, 

 together with small pieces of other rocks ejected from the crater, 

 fall down upon the land or into the sea. Here they often become 

 mingled with shells, and are stratified. Such tuffs are some- 

 times bound together by a calcareous cement, and form a stone 

 susceptible of a beautiful polish. But even when little or no lime 

 is present, there is a great tendency in the materials of ordinary 

 tuffs to cohere together. 



Besides the peculiarity of their composition, some tuns, or 

 volcanic grits, as they have been termed, differ from ordinary 

 sandstones by the angularity of their grains. When the frag- 

 ments are coarse, the rock is styled a volcanic breccia. Tufa- 

 ceous conglomerates result from the intermixture of rolled frag- 

 ments or pebbles of volcanic and other rocks with tuff. 



According to Mr. Scrope, the Italian geologists confine the 

 term tvff, or tufa, to felspathose mixtures, and those composed 

 principally of pumice, using the term pepcrino for the basaltic 

 tuffs.f 



We meet occasionally with extremely compact beds of volca- 

 nic materials, interstratified with fossiliferous rocks, much re- 



* G. Rose, Ann. des Mines, torn. 8. p. 32. 

 tGeol. Trans, vol. ii. p. 211. Second Series. 



