54 
ECONOMIC GEOLOGY 
Bauxite is therefore chiefly developed on tropical plains and 
low plateaus, and it is often associated with laterite ; for the 
ground-water dissolves iron and deposits it at the surface 
when it evaporates during the dry season. This process 
forms a bed of laterite overlying bauxite, which passes 
through the stage of lithomarge into the bedrock. 
A second mode of bauxite formation is, as at les Baux 
(Fig. 47), by the action on clay of sulphuric acid set free 
from decomposed pyrites; the acid acts on the silicate of 
alumina and forms alum (a double sulphate of aluminium 
and potassium) and aluminium sulphate ; they are carried 
up into the limestone, and deposited as veins and pockets 
of bauxite. A third mode is by the action of alkaline solutions 
raterite 
, Bauxite 
ihre 
Redeposite 
auxite 
Shale 
Slate 
Latenite 
Bauxite 
Limestone with 
pockets of Bauxite 
Pyritic Shale 
Fi16. 47.~D1AGRAM OF THE VARIOUS PROCESSES OF BAUXITE 
FormaTiON, 
Vein due to ascending solution. Pockets of bauxite in limestone due to 
sulphuric acid rising from pyritic shale. Surface action forming 
sheets of bauxite beneath laterite and overlying lithomarge. In the 
left part of the section is a bed of redeposited sedimentary bauxite. 
which, rising up fault planes, act on the wall rocks, remove 
the silica, and leave a vein of alumina. 
Secondary or detrital bauxite is formed by the washing of 
primary bauxite into lakes, where it is deposited in beds 
associated with ordinary clay, and by residual nodules of 
bauxite being left on the removal bv solution of a bed of 
limestone. 
[n the British Isles the best-known .bauxites occur in 
sedimentary beds between two series of basalts in north- 
sastern Ireland; the bauxite is of low grade, mostly contain- 
ing under 52 per cent. of alumina, and both silica and titanium 
are often high. In Scotland, near Saltcoats, pisolitic bauxitic 
clay containing from 35 to 52 per cent. of alumina has been 
formed by the alteration of Carboniferous volcanic ash.