Their monotony is varied only by the fruitful river-valleys and poljes, or upland hollows, where the smaller towns and villages are grouped; the districts or cantons thus formed are walled round by a natural rampart of limestone.
These poljes may be described as oases in what is otherwise a desert expanse of mountains.
Along the whole northern rim of Bosnia, as also in the fluvial and Karst valleys (poljes), are found diluvial and alluvial formations, interrupted at one place by an isolated granite layer.
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