THE FIELD NATURALIST'S KNOWLEDGE. 67
thick forests of the tropics, are equally at home, as are the Oxen in
our rich long-grassed meadows, and the Sheep on our fine-grassed
downs. The Monkeys live and gambol, and play their pranks in
nut-abounding forests, and equally do the Camels luxuriate in sandy
deserts where the occasional oasis furnishes their favourite prickly
though succulent herbage, even as our native swine are happy in
the woods where acorns and beech-mast plentifully sprinkle the
ground.
GEOLOGY AND PHYSIOGRAPHY.
The connection between Geology and Physiography is perhaps the
most obvious of all, for on the geological structure of a district must
depend its physical features. Hard rocks give an elevated or a
mountainous country, and soft rocks a low or flat one, and an
intermixture of hard and soft rocks will produce by the agency
of the denuding forces of Nature hills and valleys, mountains and
plains.
Again, mountains in the interior of a country will attract the
vapours of the atmosphere, and these condensed into rain and
snow will give rise to rivers and glaciers that will sculpture the
surface by cutting deep valleys and narrow gorges and ravines.
When the distance of the mountains to the sea is small the rivers
will be small, and when the distance is great the rivers will be
correspondingly great also, as is well seen on the western and eastern
sides of the South American Andes. When lofty mountains are
absent in a hill-encircled far interior, the central lands are dry and
barren, as in the centre of Australia, and when the middle of a
continent is screened from vapour-charged winds by lofty mountain
ranges it is an almost rainless area, as Thibet and Mongolia.
If the mountains are formed of hard metamorphic or granitic
rocks their summits will be peaks of more or less acuteness; if of
Cambrian or Silurian rocks the mountains, though lofty, will have
rounded tops, while ridges or "edges" will often characterise lower
elevations, as Wenlock Edge and Benthal Edge. If of Carboniferous
rocks the elevations will be low mountains, as are ours of the Pennine
Chain and the north of Derbyshire ; if of Devonian or Old Red
Sandstone rocks the elevations will be no longer mountains but more
or less bold hills with steep sides, and consequent somewhat deep
intermediate valleys.
The Secondary limestones, the Lias, the Oolites and the Chalk
will form a country of flat-topped hills with scarped sides, or smoothly
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