CHAP. Ill 
STRUCTURES OF LAVAS 
19 
even by its aid. Two varieties of devitrification may be observed among 
lavas, which, though not marked off from each other by any sharp lines, are 
on the whole distinctive of the two great groups of acid and basic rocks. 
(1) Among the acid rocks, what is called the Felsitic type of devitri- 
fication is characteristic. Thus, obsidians pass by intermediate stages from a 
clear transparent or translucent glass into a 
dull flinty or horny mass. When thin slices 
of these transitional forms are examined under 
the microscope, minute hairs and fibres or 
trichites, which may be observed even in the 
most perfectly glassy rocks, are seen to in- 
orease in number until they entirely take 
the place of the glass. JMicrolites of definite 
minerals may likewise be observed, together 
With indefinite granules, and the rock finally 
lieeomes a rhyolite, felsite or allied variety 
(Fig. 3). 
At the same time it should be observed 
that, even in the vitreous condition of a 
lava, definite crystals of an early consolidation were generally already 
present. Felspars and quartz, usually in large porphyritic forms, may 
he seen in the glass, often so corroded as to indicate that they were in 
course of being dissolved in the magma at the time of the cooling and solidi- 
fication of the mass. In obsidians and pitchstones such relics of an earlier 
cr derived series of crystallized minerals may often be recognized, while in 
Iclsites and quartz-porphyries they are equally prominent. Where large 
dispersed crystals form a prominent characteristic in a rock they give rise 
what is termed the Porphyritic structure. 
Accompanying tlie passage of glass into stone, various structures make 
Fig. 3. — Microlites of tlie Pltclistone of 
Arran (magnified 70 diameters). 
^•'“t’erlitic structure in Felsitic 
^lass, Isle of Mull (magnified). 
Fig. 5. — Spliemlitic structure 
(inagnitied). 
mr appearance, sometimes distinctly visible to the naked eye, at other 
mes only perceptible with the aid of the microscope. One of these struc- 
known as Perlitic (Fig. 4), consists in the formation of minute cui'ved 
