RECENT AND POST-PLIOCENE VOLCANIC ROCKS. 
two to five or six feet wide, and seven or eight feet deep. 
They were often hid from the sight by a quantity of scoriae 
that had formed a crust over them; and the lava, having 
been conveyed in a covered way for some yards, came out 
fresh again into an open channel. After an eruption, I have 
walked in some of those subterraneous or covered galleries, 
which were exceedingly curious, the sides, top, and bottom 
being loorn perfectly smooth and even in most parts by the 
violence of the currents of the red-hot lavas which they 
had conveyed for many weeks successively.” I was able to 
verify this phenomenon in 1858, when a stream of lava issued 
from a lateral cone.* Now, the walls of a vertical fissure, 
through which lava has ascended in its way to a volcanic 
vent, must have been exposed to the same erosion as the sides 
of the channels before adverted to. The prolonged and uni¬ 
form friction of the heavy fluid, as it is forced and made to 
flow upward, can not fail to wear and smooth down the sur¬ 
faces on which it rubs, and the intense heat must melt all 
such masses as project and obstruct the passage of the in¬ 
candescent fluid. 
The rock composing the dikes both in the modern and an¬ 
cient part of Vesuvius is far more compact than that of ordi¬ 
nary lava, for the pressure of a column of melted matter in 
a fissure greatly exceeds that in an ordinary stream of lava; 
and pressure checks the expansion of those gases which give 
rise to vesicles in lava. There is a tendency in almost all 
the Vesuvian dikes to divide into horizontal prisms, a phe¬ 
nomenon in accordance with the formation of vertical col¬ 
umns in horizontal beds of lava; for in both cases the divis¬ 
ions which give rise to the prismatic structure are at right 
angles to the cooling surfaces. (See above, p. 510.) 
Auvergne, — Although the latest eruptions in central 
France seem to have long preceded the historical era, they 
are so modern as to have a very intimate connection with 
the present superficial outline of the country and with the 
existing valleys and river-courses. Among a great number 
of cones with perfect craters, one called the Puy de Tartaret 
sent forth a lava-current which can be traced up to its crater, 
and which flowed for a distance of thirteen miles along the 
bottom of the present valley to the village of Nechers, cov¬ 
ering the alluvium of the old valley in which were preserved 
the bones of an extinct species of horse, and of a lagomys 
and other quadrupeds all closely allied to recent animals, 
while the associated land-shells were of species now living, 
such as Cyclostoma elegans,, Helix hortensis,^ H nemoralis,^ H 
* Principles of Geology, vol. i., p. 626. 
