A key part of the text that responds to this question directly is Marlowe's account of his trip to Africa on a French steamer (beginning "I left in a French steamer, and she called in every blamed port they have our there...").
The immensity of Africa is described: "The edge of a colossal jungle, so dark-green as to be almost black...". Africa is contrasted with man's ability to penetrate its depths. Any signs of humanity on its shore are insignificant in comparison with the vastness of the unknown terrain beyond: "Here and there grayish-whitish specks showed up clustered inside the white surf, with a flag flying above them perhaps. Settlements some centuries old, and still no bigger than pinheads on the untouched expanse of their background."
In response to colonial activity, the coast and Africa appears unchanged: "every day the coast looked the same". However, the image that most strongly conveys the futility of the imperial presence is the picutre of the man-of-war who was just shelling the bush. Marlowe wryly comments, "It appears the French had one of their wars going on thereabouts". This scene is described as "incomprehensible", and the shots the ship are firing are described in such a way to exemplify the futility of such actions with words such as "pop", "small flame", "little white smoke", "tiny projectile", "feeble screech". As Marlowe concludes this paragraph, we cannot help but agree with him that "There was a touch of insanity in the proceeding...", perhaps his verdict on the imperial presence in Africa.