I think part of your problem, at least as manifested by your inability to see just how and why Mark's statement is humorous, is that you simply don't understand register-jumping.
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In America, if you jump over to the register you get shot. For some reason I've been thinking of Ruggles and his eccentric symphonic piece "The Sun-Treader," which as almost everybody knows is what the hero-worshipping boy Browning called Percy Shelley in his later disowned anonymous Pauline (1833). Ruggles also set most of Blake if memory serves that also stands and ruminates. For those who do not know Ruggles, ta-da: