THE SCOPE OF ECONOMIC GEOLOGY 5 
found. The nickel ore of Sudbury and the silver veins of 
Cobalt were both cut during railway construction. Neverthe- 
less most important mining fields have been discovered by 
Systematic search. When a mining prospector visits an 
unknown country he would regard iron ore, coal, oil shale, or 
ordinary earthy minerals as useless until the country has 
been settled ; he would look for gems and the more valuable 
Metals, such as gold, platinum, tin, and copper. His search 
would usually begin by examining the local gravels, If 
their pebbles consist of unaltered limestone or sandstone 
the indication would be unfavourable ; but schists, traversed 
by quartz-veins, and abundant quartz pebbles would be 
encouraging. The prospector would use a * tin-dish” 
which is a basin of tinned iron with a flat base, and a rib on 
one side. The usual size js I6 to 20 inches in diameter at 
the top, 10 to 12 inches at the bottom, and 2} to 3 inches 
deep ; it holds, when heaped up, about 20 to 25 lb. of earth, 
which is washed by pouring water into the dish, stirring the 
gravel by hand and throwing aside the pebbles as they 
are cleaned of sang and mud. The prospector pours out 
the water as often as it becomes muddy, and at the same time 
gets rid of the fine sand until only the heavier material is left. 
This residue is spread by a flow of water along one side of the 
floor of the dish; the constituents are left in a streak in 
order of their specific gravity, the heaviest at one end and 
the lightest at the other. If gold is present it is conspicuous 
by its bright color, and as, owing to its malleability, tiny 
particles are hammered into broad flakes which are known as 
colors. They are too small to be weighed in the field, but 
On an average 35 ‘‘ colors” weigh one grain. As the tin 
dishful is from 155 10 thy of a cubic yard the prospector, by 
counting the colors, can calculate the amount of gold per 
cubic yard or ton. The other materials in the residue would 
probably include crystals of zircon, heavy silicates, and 
grains of “black sand,” ie. oxide of iron. The glassy 
grains would be examined for gems, and would be tested by 
Pressing beneath a piece of wood against a piece of glass or 
quartz. If a grain scratches quartz it is probably one of 
the gems. 
If this washing yielded nothing of value the prospector 
would follow a river inland, testing the gravels along its