英文不是靠翻译理解的！
In the course of a few hundred miles reaches the true ocean floor at 2,500-3,500 fathoms.
The true ocean floor reaches at 2,500-3,500 fathoms, in the course of a few hundred miles.
and near its lower reaches
there is a long tailing-off
which is almost certainly the result of material
transported out to deep water
after being eroded from the continental masses
Done.
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