Quote 145
"In the steady buzz of flies the homeward-bound agent was lying finished and insensible; the other, bent over his books, was making correct entries of perfectly correct transactions; and fifty feet below the doorstep I could see the still tree-tops of the grove of death." (1.48)
Marlow draws our attention to the madness of the situation by juxtaposing two very different images together—one of a man lying on his deathbed, and another of the accountant quietly going about his business as if nothing were wrong.
Quote 146
"I avoided a vast artificial hole somebody had been digging on the slope, the purpose of which I found it impossible to divine. It wasn't a quarry or a sandpit, anyhow. It was just a hole. It might have been connected with the philanthropic desire of giving the criminals something to do. I don't know. Then I nearly fell into a very narrow ravine, almost no more than a scar in the hillside." (1.38)
This random hole is another sign that people go crazy in the wilderness. (Maybe it's aliens?) But it isn't harmless eccentricity—when Marlow almost falls into another little ravine, we get the feeling that the madness is getting dangerous.
Quote 147
"For a time I would feel I belonged still to a world of straightforward facts; but the feeling would not last long. Something would turn up to scare it away. Once, I remember, we came upon a man-of-war anchored off the coast. There wasn't even a shed there, and she was shelling the bush. It appears the French had one of their wars going on thereabouts. Her ensign dropped limp like a rag; the muzzles of the long six-inch guns stuck out all over the low hull; the greasy, slimy swell swung her up lazily and let her down, swaying her thin masts. In the empty immensity of earth, sky, and water, there she was, incomprehensible, firing into a continent. Pop, would go one of the six-inch guns; a small flame would dart and vanish, a little white smoke would disappear, a tiny projectile would give a feeble screech—and nothing happened. Nothing could happen. There was a touch of insanity in the proceeding, a sense of lugubrious drollery in the sight; and it was not dissipated by somebody on board assuring me earnestly there was a camp of natives—he called them enemies!—hidden out of sight somewhere." (1.30)
Marlow is going down the rabbit hole here. One of the first acts of madness he sees is a man-of-war firing at a totally empty coastline—which sounds pretty crazy to us, too.