ECONOMIC GEOLOGY 
and in the latter includes some constituents, which the 
chemist classifies as metallic. 
The economic geologist has two special difficulties in 
addition to those which attend other branches of geology. 
Mining destroys the ores and the evidence as to their for- 
mation. A shallow mine may work out a body of rich ore 
and remove the clues to its extension underground and its 
method of formation. Unless the evidence as to the origin 
of an ore is recorded and samples preserved while it is being 
worked, its contribution to the genesis of ore deposits is lost 
for ever. The second difficulty is the unusual complexity 
of the problems; their treatment by rule of thumb often 
ends in financial loss or structural disaster. Each problem 
must be investigated by reference to the principles of geology, 
of which sound knowledge is indispensable in the economic 
applications of the science. 
THE Sporapic DisTrIBUTION oF OrES—The first striking 
feature in the study of ores is their scarcity. They occur as 
small bodies, separated by wide oreless interspaces. The 
patches of ores may be so small that if marked on a true 
scale they would be barely visible on a map of the country, 
West Australia owes its development to its rich ores; but 
they occur at widely scattered localities. The United States 
is predominant in the supply of copper. It has often yielded 
over 60 per cent. of the world’s output; and of this amount, 
in 1895, nearly half came from an area of 2 square miles 
at Butte City. The world was long dependent for aluminium 
upon a single vein of cryolite in southern Greenland, for 
potash upon central Germany, for nickel on New Caledonia 
until Sudbury in Canada shared the monoply, for mercury 
on the mining fields of Almaden in Spain, Idria near the head 
of the Adriatic Sea, and California, and for platinum on the 
Ural Mountains. 
This sporadic distribution of the mining fields is repeated 
on a smaller scale for the ore within them, which may be 
limited to one vein or ore-body, and perhaps to a small part 
of one vein; while many neighbouring veins though appar- 
ently similar may be barren. 
ProspEcTING—The sporadic distribution of ores may 
appear at first sight to render their discovery possible only 
oy accident, and some important mineral fields were thus