Sentence:  There is a water-break formed by a small terrace of rock in mid-stream, and purling with a hollow, delicious monotone—an island of pebbles is above, with here and there smaller ones near the "forks." This pebbly island is directly on the <WSD>runway</WSD>, or customary trail which the hunted deer pursues through the forests. ... I fix my eyes steadily upon the <WSD>runway</WSD>—straining them almost from their sockets—still no deer. 
Meaning of the word marked with <WSD> tags:  The usual path taken by deer or other wild animals, such as from a forest to a water source.