VOLCANIC PRODUCTS. 119 
We sometimes find sneh tuflfs 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 
hreccia, 
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 Mediterra nean, 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 rockSi 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 pitc hs tone f 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 
