216 DYNAMICAL GEOLOGY.  [Boox III. — 
perfect crater are of various volcanic rocks, many of them being — 
black scorie ; but pieces of Roman pottery, together with fragments 
of the older underlying tuff, and some marine shells, have been 
obtained—doubtless part of the soil and subsoil dislocated and 
ejected during the explosions. 
It is not necessary, and it does not always happen, that any 
actual solid or liquid volcanic rock is erupted by explosions that 
shatter the rocks through which the funnel passes. Thus among 
the cones of the extinct volcanic tract of the Eifel, some occur con- 
sisting entirely, or nearly so, of comminuted debris of the surrounding 
Devonian greywacke and slate through which the various voleanic 
vents have been opened (see pp. 206, 243). Evidently in such cases 
only elastic vapours forced their way to the surface ; and we see what 
probably often takes place in the early stages of a volcano’s history,. 
though the fragments of the underlying disrupted rocks are in most 
instances buried and lost under the far more abundant subsequent 
volcanic materials. Sections of small ancient volcanic necks or pipes 
sometimes afford an excellent opportunity of observing that these 
orifices were originally opened by the blowing out of the solid crust 
and not by the formation of fissures, Examples will be cited in later 
pages from Scottish volcanic rocks of Old Red Sandstone, Carboni- 
ferous, and Permian age. ‘The orifices are there filled with frag- 
mentary materials wherein portions of the surrounding and underlying 
rocks form a noticeable proportion.? | 
Showers of Dust and Stones.—A communication having been 
opened, either by fissuring or explosion, between the heated interior 
and the surface, fragmentary materials are commonly ejected from it, 
consisting at first mainly of the rocks through which the orifice has 
been opened, afterwards of voleanic substances. In a great eruption — 
vast numbers of red-hot stones are shot up into the air, and fall back 
partly into the crater and partly on the outer slopes of the cone. 
According to Sir W. Hamilton, cinders were thrown by Vesuvius, 
during the eruption of 1779, toa height of 10,000 feet. Instances are 
known where large stones, ejected obliquely, have described huge 
parabolic curves in the air, and fallen at a great distance. Stones 
Slb.in weight occur among the ashes which buried Pompeii. The 
volcano of Antuco in Chili is said to send stones flying to a distance 
of 36 (?) miles, and Cotopaxi is reported to have hurled a 200-ton 
block 9 miles,’ | 
Dut in many great eruptions, besides a constant shower of stones 
and scorie, a vast column of exceedingly fine dust rises out of the 
erater, sometimes to a height of more than a mile, and then spreads 
outwards like a sheet of cloud. So dense sometimes is this dust-cloud 
that the sun may be obscured, and for days together the darkness of — 
night may reign for miles around the volcano. In 1822, at Vesuvius, 
the ashes not only fell thickly on the villages round the base of the 
' Trans. Roy, Soc. Edin. xxix. p. 458. 
* D. Forbes, Geol. Mag. vii. p. 320. 


