THE FORMATION OF DEPOSITS 33 
‘posited ; and which of them are primary and which are 
iecondary, It also shows which of the minerals In an ore 
Vere deposited by the agent that introduced the metals, 
and whether the minerals are still as they were originally 
deposited or whether they have been redeposited by de- 
ending surface wagers, It also reveals in replacement ores, 
% the shadow of the original structure, what material the 
ore has replaced ; it may show, for example, that a phosphate 
of lime wag originally a trachyte (as at Clipperton Island) ; 
that a gold-bearing quartzite was originally a dolomite or 
a fossiliferoysg limestone ; that an ironstone has been formed 
by the alteration of a shelly limestone into a carbonate or 
oxide of iron, Many theories have proved untenable when 
microscopic study has shown that supposed sedimentary 
rocks were igneous, or that intrusive rocks were volcanic 
ash, and that a mineral that was thought to have been the 
frst constituent tq solidify in a molten magma was intro- 
duced long after the solidification of the rock. 
CHIEF Minera Deposits puz 10 SEGREGATION—Most of 
“he minerals of special use tq man consist of material that 
was widely scattered through the primeval matter of the 
®arth. They have been concentrated by that beneficent 
Process of Segregation which draws like to like. The process 
® sometimes dye to chemical affinity, and sometimes to the 
Toperties which Cause mechanical concentration by wind 
And water, The Primary mineral deposits are mainly due 
to ascending currents rising from the vast store of metals 
'n the interior of the earth. Some rich secondary deposits 
re made upon or near the surface by the solution of scattered 
a descending rain-water, and its deposition in 
“oncentrated form. Some superficial deposits are due to 
the mechanical Separation of ore from dross by Nature's 
use of the methods adopted in ore-concentrating machinery. 
Other beds of mineral matter, such ag coal and limestone, 
are due to the gregarious habits of plants and animals ; 
they live in forests or jungles or in colonies, and leave their 
“Ssues, shells op skeletons in continuous sheets which are 
subsequently Compressed and cemented. Varied processes 
of concentration have formed all thoge mineral segregations 
>Y the use of which man has gained his control over nature, 
and developeq modern civilization,