M&D Deep Duck Read. Philadelphia

[Mark Kohut]

By the 1750s, Philadelphia had surpassed Boston to become the largest
city and busiest port in British America, and second in the British
Empire, behind London.[5][6]

P 6. the City.. 'Snow lies upon all Philadelphia .'behind curtains of
ice-fog that the City lies like .'an Isle upon an Ocean'.........

Why such a metaphor here?...why want an image as an Isle upon an Ocean?...
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Pynchon-l / http://www.waste.org/mail/?list=pynchon-l