ECONOMIC GEOLOGY 
the pure water necessary to its health. In addition to the 
study of these materials, the economic geologist is concerned 
with the arrangement of rocks to facilitate their quarrying, 
with the protection of coasts from attack by the sea, of 
plains from devastation by rivers, of harbours from shoaling, 
and of buildings from overthrow by earthquakes, with the 
avoidance of hidden dangers in the selection of reservoir 
sites, and the maintenance of public health by the utilization 
of underground water and safe methods of drainage. 
The problems of economic geology are complex owing to 
the multiplicity of the materials, the variability of local 
conditions, and the influence of prices and costs. A material 
which in one place may be a valuable ore in another may be 
commercially worthless. Profitable use is an essential factor 
in the definition of the term ore. An ore is a material con- 
taining sufficient metal to be worth mining under conditions 
which either already exist at the locality, or may be reason- 
ably expected. 
MiNERAL—This term is used with two different meanings. 
Mineral in the general sense is any inorganic material, and 
includes animal and vegetable products which have been 
buried in the earth and become part of its crust. Some 
geologists limit the term to materials which have a definite 
chemical composition, and usually a regular crystalline shape, 
and regard coal, slate, limestone, mineral oil, oil shale, and 
most ores as not minerals. The aim of this inconvenient 
definition is to emphasize the distinction between simple 
minerals and rocks. Before about 1850, rocks were regarded 
as minerals ; the first editions of Dana’s System of Mineralogy 
included a chapter on “Rocks or Mineral Aggregates.” 
Lyell (Principles of Geology, 7th ed., 1847, p. 784) distin- 
guished between * simple minerals ” and mineral aggregates. 
Minerals were divided into two sections; simple minerals, 
or mineral species, such as quartz and felspar, cannot be 
divided into simpler constituents without chemical decom- 
position ; compound or mixed minerals may be separated 
into their components mechanically, as granite can be separ- 
ated into its three mineral species, quartz, felspar, and 
mica, by crushing and sorting the fragments. As academic 
mineralogy developed it was limited to the study of mineral 
species, and most compound minerals were left to the branch