[sariputra]

Originally Posted by Mister Micawber

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This is impossible: nouns cannot be subjects of other nouns (a gerund is a noun).

The whole clause ('the ton of books falling on them both'), however, is a noun clause and the object of the preposition 'without'.) That would, I suppose, make 'falling' here a gerund. It more clearly appears so with the more formally styled 'the ton of books' falling...'.
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I am very grateful to you, Mister Micawber, for your kind reply. It is very helpful to me.