"I was thinking of very old times, when the Romans first came here, nineteen hundred years ago—the other day. . . . Light came out of this river since—you say Knights? Yes; but it is like a running blaze on a plain, like a flash of lightning in the clouds. We live in the flicker—may it last as long as the old earth keeps rolling! But darkness was here yesterday. Imagine the feelings of a commander of a fine—what d'ye call 'em?—trireme in the Mediterranean, ordered suddenly to the north; run overland across the Gauls in a hurry; put in charge of one of these craft the legionaries,—a wonderful lot of handy men they must have been too—used to build, apparently by the hundred, in a month or two, if we may believe what we read. Imagine him here—the very end of the world, a sea the color of lead, a sky the color of smoke, a kind of ship about as rigid as a concertina—and going up this river with stores, or orders, or what you like. Sandbanks, marshes, forests, savages,—precious little to eat fit for a civilized man, nothing but Thames water to drink. No Falernian wine here, no going ashore. Here and there a military camp lost in a wilderness, like a needle in a bundle of hay—cold, fog, tempests, disease, exile, and death,—death skulking in the air, in the water, in the bush. They must have been dying like flies here. Oh yes—he did it. Did it very well, too, no doubt, and without thinking much about it either, except afterwards to brag of what he had gone through in his time, perhaps. They were men enough to face the darkness. And perhaps he was cheered by keeping his eye on a chance of promotion to the fleet at Ravenna by-and-by, if he had good friends in Rome and survived the awful climate. Or think of a decent young citizen in a toga—perhaps too much dice, you know—coming out here in the train of some prefect, or tax-gatherer, or trader even, to mend his fortunes. Land in a swamp, march through the woods, and in some inland post feel the savagery, the utter savagery, had closed round him,—all that mysterious life of the wilderness that stirs in the forest, in the jungles, in the hearts of wild men. There's no initiation either into such mysteries. He has to live in the midst of the incomprehensible, which is also detestable. And it has a fascination, too, that goes to work upon him. The fascination of the abomination—you know. Imagine the growing regrets, the longing to escape, the powerless disgust, the surrender, the hate."
Full text available at The Project Gutenberg: Heart of Darkness