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"The brown current ran swiftly out of the heart of darkness, bearing us down towards the sea with twice the speed of our upward progress; and Kurtz's life was running swiftly, too, ebbing, ebbing out of his heart into the sea of inexorable time." (3.36)
Going downstream on the Congo River, which we have by now equated with traveling through time, is much faster than moving upstream. Since Marlow and his crew are headed back towards civilized Europe, they feel as if they are traveling forward in time.
"All that had been Kurtz's had passed out of my hands: his soul, his body, his station, his plans, his ivory, his career. There remained only his memory and his Intended – and I wanted to give that up, too, to the past, in a way – to surrender personally all that remained of him with me to that oblivion which is the last word of our common fate." (3.50)
One of the reasons Marlow wants to get rid of Kurtz’s letters is so that he can put Kurtz and his whole journey behind him, so that he can resign it peacefully to the past.
"I thought his memory was like the other memories of the dead that accumulate in every man's life – a vague impress on the brain of shadows that had fallen on it in their swift and final passage; but before the high and ponderous door, between the tall houses of a street as still and decorous as a well-kept alley in a cemetery, I had a vision of him on the stretcher, opening his mouth voraciously, as if to devour all the earth with all its mankind. He lived then before me; he lived as much as he had ever lived – a shadow insatiable of splendid appearances, of frightful realities; a shadow darker than the shadow of the night, and draped nobly in the folds of a gorgeous eloquence. The vision seemed to enter the house with me - the stretcher, the phantom-bearers, the wild crowd of obedient worshippers, the gloom of the forests, the glitter of the reach between the murky bends, the beat of the drum, regular and muffled like the beating of a heart – the heart of a conquering darkness. It was a moment of triumph for the wilderness, an invading and vengeful rush which, it seemed to me, I would have to keep back alone for the salvation of another soul. And the memory of what I had heard him say afar there, with the horned shapes stirring at my back, in the glow of fires, within the patient woods, those broken phrases came back to me, were heard again in their ominous and terrifying simplicity. I remembered his abject pleading, his abject threats, the colossal scale of his vile desires, the meanness, the torment, the tempestuous anguish of his soul. And later on I seemed to see his collected languid manner, when he said one day, 'This lot of ivory now is really mine. The Company did not pay for it. I collected it myself at a very great personal risk. I am afraid they will try to claim it as theirs though. H'm. It is a difficult case. What do you think I ought to do - resist? Eh? I want no more than justice.' […] He wanted no more than justice - no more than justice. I rang the bell before a mahogany door on the first floor, and while I waited he seemed to stare at me out of the glassy panel – stare with that wide and immense stare embracing, condemning, loathing all the universe. I seemed to hear the whispered cry, "The horror! The horror!" (3.51)
The past comes powerfully and vividly alive for Marlow as he makes his way to the Intended’s house. He finds that his memories are not the "vague impress on the brain" that he has been accustomed to. In contrast, he remembers his meeting with Kurtz quite lucidly.