Distinct from all the rest of the southern highlands of Europe stands
the huge mass of the _Caucasus_, the natural frontier of Europe on the
southeast, rising like a wall from the flat isthmus between the Black
Sea and the Caspian. Its close parallel chains are united by high
plateaus cut into by deep narrow transverse gorges of extreme depth.
Though attaining far greater heights than the Alps and reaching several
thousand feet above the limit of perennial snows, the glaciers and
snow-fields of the Caucasus are small and insignificant in comparison
with those of the Alps. This is owing to the dryness of the region in
which they stand, and the small snowfall over them.