in Thoreau’s Journal:

A pure brook is a very beautiful object to study minutely.  It will bear the closest inspection, even to the fine air-bubbles, like minute globules of quicksilver, that lie on its bottom. The minute particles or spangles of golden mica in these sands, when the sun shines on them, remind one of the golden sands we read of. Everything is washed clean and bright, and the water is the best glass through which to see it….