Sentence:  The circle of <WSD>weltering</WSD> froth at the base of the Horseshoe, emerging from the dead white vapours—absolute white, as moonless midnight is absolute black—which muffle impenetrably the crash of the river upon the lower bed, melts slowly into the darker shades of green. 
Meaning of the word marked with <WSD> tags:  To rise and fall, to tumble over, to roll.