How does Marlow's statement of feeling like he was going to the center of the earth relate to the following statement?
As Marlow observed, "I felt as though, instead of going to the center of a continent, I were about to set off for the center of the earth." Consider how this statement relates to the following statement: "...the archetypal myth dramatized in much great literature since the book of Jonah: the story of an essentially solitary journey involving profound spiritual change in the voyager. In its classical form, the journey is a descent into the earth, followed by a return to light."
The statement you quote regarding the archetypal myth sounds like something that could have been written by Joseph Campbell. I am impressed by your acuity. I have read some of Joseph Campbell's writings and listened to him on television, but the connection between him and Heart of Darkness never occurred to me. Of course, Conrad came a long time before Campbell, so Heart of Darkness would have to be considered another example of the stories the quoted statement refers to. If you are thinking of writing a paper based on this comparison, I think it would be an excellent idea. The comparison offers an explanation of Conrad's story which is much deeper and more intriguing than just a description of Africa under imperialism. It seems to explain why Heart of Darkness creates such a strong effect on the reader. I hope you develop your idea.