
\ - ~ Sine es 
142 | @EOGINOSY. Bete a 
Perlite is a markedly acid rock, its percentage of silica ranging a 
between 70°6 and 82'8, and its average specific gravity between 2°37 
and 2°46. It occurs most conspicuously in Hungary, where it takes 



































Fic. 22.—Microscorico STRUCTURE Fig. 23.—Microscopic STRUCTURE 
oF PERLITE. oF SPHERULITE-ROOK. 
the form of lava streams proceeding from old trachyte volcanoes ; 
also among the Euganean Hills, Ponza Islands and Ascension.’ 
Pumice (Ponce, Bimstein).—A general term for the loose, spongy, 
cellular, filamentous or froth-like parts of lavas. So distinctive is this 
structure, that the term pwmiceous has come into general use to 
describe it. There can be no doubt that this froth-like rock owes its 
peculiarity to the abundant escape of steam or gas through its mass 
while still in a state of fusion. Microscopic examination reveals a 
glass crowded with enormous numbers of minute gas or vapour 
cavities usually drawn out in one direction, also abundant crystallites 
like those of obsidian. In the great majority of cases pumice is a 
form of the obsidians, possessing a percentage of silica from 58. to 
74, and a specific gravity of 2:0 to 2:53, though, owing to its porous 
nature, it possesses great buoyancy and readily floats on water, 
drifting on the ocean to distances of many hundreds of miles from 
land, until Pe cells are gradually filled with water, when the floating 
masses sink to the bottom.?, Abundant rounded blocks of pumice 
were dredged up by the Challenger from the floor of the Atlantic and 
Pacific Oceans. At Hawaii, some of the basic pyroxenic or olivine 
lavas give rise to a pumiceous froth. 
xX 
\ 
b. Plagioclase Rocks. 
The rocks of this division are of all ages up to the present 
time. ‘They consist essentially of some triclinic felspar to which 
one, more usually several other silicates are added. As a rule 
‘ Mr. Allport has described some ancient forms of perlite from Shropshire, Q. J. 
Geol. Soc. xxxiii. p. 449; and Mr, Rutley has shown the presence of perlitic structure 
among the Lower Silurian lavas of North Wales. Op. cit. xxxv. p. 508. 
? On porosity, hydration, and flotation of pumice, see Bischof, Chem. und Phys. Geol. 
Suppl. (1871) p. 177. 
