VOLCANIC PRODUCTS. 



119 



We sometimes fir.d 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 iheir 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 ivacke, 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 whinsfone, 

 which is a Scotch provincial term for greenstone 

 and other hard trap rocks : also pitchslone, which is 

 a vitreous lava, less glassy thati 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 



