Explanation: A simile is a direct comparison between elements that aren't obviously related, it helps the reader to form a mental image and to understand a point. In the given lines we can see two examples of simile, the first one is "the river like the silver track left by a snail" which compares the river with the track left by a snail, and the second one is "and roads as narrow as ribbons" which compares the road with ribbons.
