Final answer:
The excerpt from Heart of Darkness expresses the idea of unchecked colonial power and intrusion onto colonized lands.
Step-by-step explanation:
The idea about colonialism most clearly expressed in this excerpt is the notion of the unchecked power and intrusion of colonizers onto colonized lands. The man-of-war shelling the bush represents the colonial power exerting force and dominance over the continent, without fully understanding or comprehending the local context. The line 'There was a touch of insanity in the proceeding' suggests that the actions of the colonizers were irrational and lacked reason.
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