A moving torrent and a fast caravan share an easily recognized quality of continuous rapid movement. That underlying association is not especially obscure.
The poet, however, does more than call the camels a flood. The force of their movement is projected across the traveled ground, making the valley beds themselves appear to flow.
The uncommon element is therefore not an unprecedented shared feature. It is the unexpected way in which a familiar shared feature is expressed.
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Familiar and Distinctive Metaphor: Obvious Resemblance and Artful Rarity
A moving torrent and a fast caravan share an easily recognized quality of continuous rapid movement. That underlying association is not especially obscure.
The poet, however, does more than call the camels a flood. The force of their movement is projected across the traveled ground, making the valley beds themselves appear to flow.
The uncommon element is therefore not an unprecedented shared feature. It is the unexpected way in which a familiar shared feature is expressed.