10 
ECONOMIC GEOLOGY 
patches. The most characteristic concentrations are ‘ ore 
shoots,” which are often irregularly funnel-shaped or cylin- 
drical bodies of ore. Such shoots may be due to concentrated 
precipitation of the metallic constituents of a solution by 
reactions with the wall rock of the lode, or with a solution 
that enters the lode fissure from some side channel. A shoot 
may be formed where a metalliferous spring enters a sheet 
of water in a fissure. Some masses of iron ore have been 
formed as igneous segregations, such as that at Taberg in 
Sweden. Iron ore bodies of greater commercial value have 
been formed as concentrations by the descent of surface 
waters, which dissolve iron from some iron-bearing rock 
and carries it down until it js stopped by impermeable 
material and is precipitated in mass. Thus have been formed 
the rich masses of kidney iron ore of the north-west of England 
{cf. p. 135) and the colossal bodies of iron ore beside Lake 
Superior. Some funnel-shaped bodies of lead ores, which 
are often stalactitic, are due to the concentration of scattered 
particles of lead by descending solutions. 
EFFLORESCENT MinerALs—Exceptionally rich mineral 
concentrations are formed near the surface as ascending water 
there undergoes rapid chemical and physical changes, The 
water may evaporate and deposit its mineral matter as a 
bed of tufa or sinter around a spring, or as a widespread 
efflorescent layer or crust. In a dry climate very soluble 
salts may be thus deposited, such as the nitrate of soda of 
Chile and Peru; in a climate with alternate wet and dry 
seasons limestone and chert may be deposited by the water 
which has soaked into the ground during the rains, being 
sucked to the surface and evaporated during the dry weather. 
Efflorescent limestones may form a nodular sheet, such as 
that which mantles the undulating surface of the Mallee 
tountry in north-western Victoria, the Kankar of India, 
and the caliche of Mexico. The Mallee limestone is inter- 
bedded with chert and ironstone where the descending rain- 
water dissolved silica or iron. Amongst the important minerals 
formed by the alternate descent of rain-water during the wet 
season, and evaporation from the surface during the dry 
season are bauxite, the chief ore of aluminium (p. 153) and 
laterite. 
Superficial ores are formed on the floors of lakes and in