VOLCANIC PRODUCTS. 



119 



We sometimes find such tuffs bound together by a 

 calcareous cement, forming a stone susceptible of 

 a beautiful polish. Some tuffs, or volcanic grits, as 

 they have been termed, differ from ordinary sand- 

 stones by the angularity of their grains. When the 

 fragments are coarse, the rock is styled a volcanic 

 breccia. 



Tufaceous conglomerates result from the inter- 

 mixture of rolled fragments or pebbles of volcanic 

 and other rocks with tuff or tufa. Mr. Lyell re- 

 marks, that the extremely compact beds of volcan- 

 ic materials, interstratified with fossiliferous rocks, 

 may be tuffs, notwithstanding their density. In 

 proof of this, it is stated that the chocolate colour- 

 ed mud which was poured out for weeks from the 

 crater of Graham's Island, in the Mediterranean, in 

 1831, must, when intermixed with other materials, 

 have constituted a stone heavier than granite. And, 

 however improbable it may appear, it has been as- 

 certained that each cubic inch of the impalpable 

 powder which has fallen for days through the at- 

 mosphere during some modern eruption, has been 

 found to weigh, without being compressed, as much 

 as ordinary trap rocks, which are often identical in 

 mineral composition. 



Besides the above, there are other varieties of vol- 

 canic rocks, such as wacke, which is a soft and 

 earthy variety of trap, having an argillaceous as- 

 pect, resembling indurated clay, and exhibiting a 

 shining streak when scratched : also whinstone, 

 which is a Scotch provincial term for greenstone 

 and other hard trap rocks : also pitchstone, which is 

 a vitreous lava, less glassy than obsidian, a black- 

 ish green rock, resembling glass, and having a res- 

 inous lustre and appearance of pitch, composed of 

 feldspar and augite, and passes into basalt. 



Igneous or volcanic rocks are generally more fu- 

 sible than others, there being much alkaline matter 

 and lime in their composition, which serves as a 



