272 Reviews and Extracts. — Jfatural History. 
that tliere exist many sub-murinc Volcanoes, of vvliich tlie island formed in 1707, 
near Santorin, those in 1638, 172i(, and 1811, near St. Michael, in the Azores, 
that in 171-3, on the south-west coast of Iceland, and other phenomena of a like 
nature, afford ample proof. 
Volcanoes do not always continue in a state of activity, but experience periods 
of cessation, and even remain quiet for centuries, tnid some have now the appear- 
ance jof bein<f finally exhausted. The first symptoms of volcanic action, are 
subterranean noises, sliakiuLjs of the g-round, and the emission of smoke by the 
crater. When the air is agitated, the smoke rolls along- in immense clouds, which 
cover the surrounding- country with darkness; jets of burning- sand and ashes from 
the Volcano traverse these columns, and clouds which sometimes emit iightuings. 
Then follow eruptions of incandescent masses of stone, which are projected into the 
air, and fall in all directions around the Volcano, in the form of sand, dust, cin- 
ders, or stones; the shaking^s of the earth continue ; the melted matter contained 
in the Volcano ascends into the crater, and overflowing the lawost part of its edge, 
descends along the sides of the mountains, sometimes with great velocity, but 
more usually in a stream flowing with slow and majestic motion. Frequently an 
opening takes place in the side of the mountain, through which the lava makes its 
escape. Enormous currents of mud and water burst forih, and torrents of rain 
fall from the atmosphere, while the noxious exhalations often spread over the sur- 
rounding country, and destroy animal and vegetable life. 
The smoke of V^olcanoes generally consists of aqueous vapour, impregnated with 
hydrogen and carbonic acid gases, or sulphuric acid. The ashes consist of pow- 
der, of a grey colour, in a state of extre.iie tenuity, and are usually mixed with 
sand. In the eruption of Hecla, in 1766, complete darkness was produced by the 
cloud of ashes to a great distance; and in 1812, a cloud of volcanic ashes and 
sand, from a Volcano in the island of St. Vincent's, covered the whole of Burlia- 
does, producing such a degree of darkness, that at noon the nearest olycct could 
not be distinctly perceived. Volcanic sand consists of small cinders, usually mixed 
with crystals of augite and felspar, and fo/ms the greater portion of the substances 
projected by Volcanoes. Scori-*, or cinders, consist of portions of the melted lava 
projected by the escape of elastic fluids. One of the most remarkable substances 
produced by Volcanoes is sulphur, which, Humboldt remarks, is very rare in cra- 
ters which are still burning, while all the old Volcanoes ultimate'y become solfa- 
terras. 
Lava is the mass of incandescent and iiielted matter, wliich carried upwards to 
ihe mouth of tlie crater by the force of the elastic fluids, escapes at the lowest 
part of tiie brim, or through an aperture formed in the side of the mountain. By 
the eruption of the Skaptar Jokul, in Iceland, in (783, two vast streams of lavu 
were produced, one of vvl.ieh was fifty, the other forty miles in length, with a 
breadth in some places exleuiling fifteen miles, and an average depth of one hun- 
dred feet. Von Buch, who witnessed the eruption of Vesuvius in 180.5, saw a 
torrent descend from the summit to near the sea, over a space of mose than seven 
thousand metres, (about four miles and a half,) in three hours. In general, how- 
ever, currents of lava move with extreme slowness ; and Dolomieu mentions one 
which took two years to traverse a space of three hundred and eighty metres. 
Tlie tenacity of lavas is us-jally so great that it is difficult to thrust a stick into 
them while they are flowing; and Spallanzani hardly produced any impression 
upon a current by throwing very large .stones upon it. Lavas take also an extra- 
ordinary time in cooling. Currents are mentioned which were still progressing 
ten years after their eruption. Spallanzani, while passing over a current which 
had ceased to tlovv for eleven inonthsj saw through the cracks in its surface that it 
still was I'.'d. 
