The Treasure of Pearls by Gustave Aimard

Upon this enjoinder of so eminently practical a nature, and thoroughly aware of the necessity of haste, the fallen Mexican rapidly drew with his ramrod end, upon a space of earth smoothed by his foot in its deerskin boot, like an antique tablet under the stylus, a map -- rude, but, to a navigator, plain and ample.

"At this point," said he, "a sunken reef trends north and south, with a break at a little bow a quarter mile from the black rock that juts out all but flush with its ripple. Deep water in 'the pot,' and there we anchored to ride to a submerged buoy, so that the cankerworm would not attack the metal or the borer the wood -- a chest, bound with yellow metal. If it shall have broke away, its weight would only have sunk it deep in the oyster bed, all the shells there smashed to powdery scales by the drags. A diver will find it for you, then."