a aor el aie dil bot 
St ae , oe dy al ie 
a ; “tS “a 
. : <3 eee 
’ cae 
294 DYNAMICAL GEOLOGY. . [Boox. TR @ 
_ proportion of iron. In this respect they resemble the natural glasses _ 
—pitchstones and obsidians. They almost always contain minute 
cells or bubbles, arising probably from the disengagement of water 
or of oxygen. But after the most thorough fusion which has been 
found possible, minute granules usually appear in the solidified — 
glass. Sometimes these consist of specks of quartz (which from 
its refractory nature is especially apt to remain unmelted) or of 
other minerals of the original rock.’ 
Microscopic investigation of artificially-fused rocks shows that, 
even in what seems to be a tolerably homogenous glass, there are 
abundant minute hair-like, feathered, needle-shaped, or irregularly- 
ageregated bodies diffused through the glassy paste. These crystal- 
lites, in some cases colourless, in others opaque, metallic oxides, 
particularly oxides of iron, resemble the crystallites observed in many 
voleanic rocks (p. 100). They may be obtained even from the fusion 
of a granitic or granitoid rock, as in the well-known case of the 
Mount Sorrel syenite near Leicester, which, being fused and slowly 
cooled, yielded to Mr. Sorby abundant crystallites, including 
exquisitely-grouped octohedra of magnetite.” 
According to the observations of Delesse, volcanic rocks, when 
reduced to a molten condition, attack briskly the sides of the 
Hessian crucibles in which they are contained, and even eat them 
through. ‘This is an interesting fact, for it helps to explain how 
some intrusive igneous rocks have come to oceupy positions previously 
filled by sedimentary strata, and why, under such circumstances, the 
composition of the same mass of rock should be found to vary 
considerably from place to place.* 
Contraction of Rocks in passing from a Glassy to a Stony 
State.—Reference has been made (pp. 284, 291) to the expansion of 
rocks by heat and their contraction on cooling ; likewise to the differ- 
ence between their volume in the molten and in the solid state. It 
would appear that this diminution in density as rocks pass from a crys- 
talline into a vitreous condition, is, on the whole, greater the more 
silica and alkali are present, and is less as the proportion of iron, lime, © 
aud alumina increases. According to Delesse, granites, quartziferous 
porphyries, and such highly silicated rocks lose from 8 to 11. 
per cent. of their density when they are reduced to the condition of 
glass, basalts lose from 3 to 5 per cent., and lavas, including the 
* One of the basalts of Arthur’s Seat, Edinburgh, after exposure toa high temperature | 
for four hours, was supposed to be completely fused; but was found on examination 
with the microscope to have retained its large labradorite crystals, only partially rounded 
on the edges and otherwise unaffected. ' 
* Zirkel, Mik. Besch. p. 92; Sorby, Address Geol. Sect. Brit. Assoc. 1880. On the 
microscopic structure of slags, &c., see Vogelsang’s “ Krystalliten.”’ 
* Bull. Soc. Géol. France, 2nd ser., iv. 1382; see also Trans. Edin. Roy. Soc. xxix. 
p- 492. Bischof has described a series of experiments on the fusion of lavas with 
different proportions of clay-slate. He found that the lava of Niedermendig kept an 
hour in a bellows-furnace was reduced to a black glassy substance without pores, and 
that a similar product was obtained even after 30 per cent. of clay-slate had been added 
and the whole had been kept for two hours in the furnace. Ohem. und Phys. Geol. 
Supp. (1871), p. 98. 
