n thickness, causing those hills to
rival, or even to surpass, in height, Salisbury Craigs and Arthur's
Seat.

The distance between the extreme points here indicated would not exceed
ninety miles in a direct line; and we might then add, at the distance of
nearly two hundred miles from London, along the coast of Dorsetshire and
Devonshire, for example, a great mass of igneous rocks, to represent
those of contemporary origin, which were produced beneath the level of
the sea, where the island of Nyoe rose up.

_Volume of ancient and modern flows of lava compared._--Yet, gigantic as
must appear the scale of these modern volcanic operations, we must be
content to regard them as perfectly insignificant in comparison to
currents of the primeval ages, if we embrace the theoretical views of
many geologists, which were not inaccurately expressed by the late
Professor Alexander Brongniart, when he declared that "aux apoques
gaognostiques anciennes, tous les phanomenes gaologiques se passoient
dans des dimensions _centuples_ de celles qu'ils prasentent
aujourd'hui."[587] Had Skaptar Jokul, therefore, been a volcano of the
olden time, it would have poured forth lavas at a single eruption a
hundred times more voluminous than those which were witnessed by the
present generation in 1783. But it may, on the contrary, be affirmed
that, among the older formations, no igneous rock of such colossal
magnitude has yet been met with; nay, it would be most difficult to
point out a mass of ancient date (distinctly referable to a single
eruption) which would even rival in volume the matter poured out from
Skaptar Jokul in 1783.

_Eruption of Jorullo in 1759._--As another example of the stupendous
scale of modern volcanic eruptions, I may mention that of Jorullo in
Mexico, in 1759. The great region to which this mountain belongs has
already been described. The plain of Malpais forms part of an elevated
platform, between two and three thousand feet above the level of the
sea, and is bounded by hills composed of basalt, trachyte, and volcanic
tuff, clearly indicating that the country had previously, though
probably at a remote period, been the theatre of igneous action. From
the era of the discovery of the New World to the middle of the last
century, the district had remained undisturbed, and the space, now the
site of the volcano, which is thirty-six leagues distant from the
nearest sea, was occupied by fertile fields of sugar-cane and indigo,