Physical geography. By Mary Somerville ...

42 PHYSICAL GEOGRAPHY. CHAP. II.
character of the country, and the insalubrity of its climate, on the
Atlantic coast at least.
The continents had been raised from the deep by a powerful effort
of the internal forces acting under widely extended regions, and the
stratified crust of the earth either remained level, rose in undulations, or sank into cavities, according to its intensity. Some thinner portion of the earth's surface, giving way to the internal forces,
had been rent into deep fissures, and the mountain masses had been
raised by violent concussions, perceptible in the convulsed state of
their strata. The centres of maximum energy are marked by the
plutonic rocks,' which generally form the nucleus or axis of the
mountain masses, on whose flanks the stratified rocks are tilted at
all angles to the horizon, whence, declining on every side, they sink
to various depths, or stretch to various distances in the plains.
Enormous as the mountain-chains and table-lands are, and prodigious as the forces that elevated them, they bear a very small proportion to the mass of the level continents and to the vast power
which raised them even to their inferior altitude. Both the high
and the low lands have been elevated at successive periods; some
of the very highest mountain-chains are but of recent geological
date, and some chains that are now far inland once stood up as
islands above the ocean, while marine strata filled their cavities and
formed round their bases. The influence of mountain-chains on the
extent and form of the continents is beyond a doubt.
Notwithstanding the various circumstances of their elevation,
there is everywhere a certain regularity of form in mountain masses,
however unsymmetrical they may appear at first, and rocks of the
same kind have identical characters in every quarter of the globe.
Plants and animals vary with climate, but a granite mountain has
the same peculiarities in the southern as in the northern hemisphere
-at the equator as near the poles. Single mountains, insulated on
plains are rare, except when they are volcanic; they generally appear
in groups intersected by valleys in every direction, and more frequently in extensive chains symmetrically arranged in a series of
parallel ridges, separated by narrow longitudinal valleys, the highest and most rugged of which occupy the centre:2 when the chain
Plutonic rocks are granite and others owing their origin to fire.
2 According to M. Elie de Beaumont, every system of mountains occupies
a portion of a great circle of the globe, the cleft being more easily made in
that, than in any other direction, and he shows that the mountain chains
are parallel to one another, even when in opposite hemispheres; thus.the
Central Alps and Carpathians, the Caucasus and Himalaya, lie nearly in the
same direction. The great circle of the sphere, that would pass through
that part of the Apennines lying between Genoa and the sources of the
Tiber, is parallel to the mountains in Achaia, to the Pyrenees, to the Alleghanies in North America, and to the Ghauts in Malabar. The Western
Alps are parallel to the Spanish mountains from Cape San Maritimo to