Treasure Maps

Bill James (Australia) writes:

An up-to-date survey map offers rock hounds as many clues to buried treasure as any pirate chart of the Spanish Main ever did.

Contour lines make up the bulk of clues. These lines indicate the rise and fall of the land surfaces by connecting points at the same level and thus outline the shapes of hills and valleys as seen from above. The lines are drawn at regular intervals and distance between them indicates the steepness of the slope. Where the lines are spaced evenly, the land is rising gradually. Where they bunch together, the rise is abrupt, as on a cliff face.

Other details are shown by symbols explained in the margin of the map. Tiny green sketches represent scattered woodland, green patches shows the heavier bush and rivers appear as wavy blue lines. Creeks which only flow in wet weather are shown in broken blue lines. Careful study of this map tells the gem seeker where he is likely to find bare rocks above talus slopes littered with fragments from the weathered face and also the places to which streams and rivers have carried debris from the rocks.

We can trace the pattern of blue lines on the hillside where a multitude of trickles feed a brook and a multitude of brooks feed a river. Study of this drainage system shows how gemstones are gathered from thousands of acres of hillside. Checking the course of the river against contours shown on the map discloses the places where the current slows down and the gem gravels pile up.

Obviously a river cannot collect gemstones unless it flows among gem-bearing rocks. To identify rocks, the survey map must be compared against a geological map of the area. On the geological map, the rocks occurring beneath the soil level are picked out in contrasting colors and named.

If the rocks of the area are granite, there is a good chance of finding beryl, feldspar, topaz, tourmaline and gem varieties of quartz. Basalt and volcanic tuffs may contain chalcedony, opal, jasper and agate, both in the form of seams and thunder eggs. Gneiss, schists and marbles yield sapphire, garnets, spinel and zircon.

But that is not to say that rocks of any given variety must necessarily be gem-bearing. Some granites, for instance, produce no gems, but in that case the survey map again provides a clue. Strong mineralized granites are the ones which becomes weathered most quickly, forming smoothly rounded hills, well covered with the trees and bushes marked on the map.

Pale streaks of quartz veins or quartz floaters may be visible on such granites. Pegmatites may outcrop, showing glittering mica surfaces in the sunshine, or rock crystals peep among the grass roots to pinpoint a pocket of gemstones.

Almost certainly the survey map will show mines in the vicinity—mines for wolfram, or tin, or silver-lead, or any one of half-a-dozen other minerals. Given permission by the owners, fossicking on mullock heaps of mines and quarries can be a rich source of gem materials.

Treasure Map: (continued)