2 
ECONOMIC GEOLOGY 
and lagoons. The most important of the organically formed 
minerals is the coal series, ranging from peat through lignite 
and bituminous coal to anthracite and graphite, and through 
cannel coal to oil shale. The asphaltic limestones owe their 
special value to their organic constituents, which when heated 
form bitumen. The phosphatic rocks are also usually of 
organic origin, their phosphoric acid being obtained from 
bird dung deposited on islets, or bones carried into lagoons. 
The phosphoric acid is carried into the underlying material 
and converts it into phosphates, of which the phosphate of 
lime is one of the most valuable fertilizers. 
ArLLuviaL Ores AND Pracers—Surface conditions also 
give rise to those sedimentary ores in which the useful con- 
stituent has been obtained from an older mineral deposit, 
which has been broken up and its grains left in beds of sand 
or gravel forming alluvial ores or placers. The term placer 
is a Spanish word used for a sand-bank and for pearl fisheries. 
The most important of these mechanically-formed ores are 
those of heavy metals which are insoluble in surface waters 
ander ordinary conditions. The chief placer deposits con- 
tain gold, platinum, tin, and the gems. Most placers are 
due to rivers, which deposit ore washed out of lodes along 
their course. Deeply buried river beds are known as “ deep 
leads.” Marine placers are formed where the surf breaks 
up metalliferous rocks and deposits the metallic grains in 
patches, as at Nome in Alaska and in New Zealand. The 
most important marine placer is that ancient gold-bearing 
shingle, the Banket of the Rand, which has proved the most 
prolific of the world’s gold ores. 
Microscopic Stuy oF OrEs—Knowledge of the deposition 
of ores and help in their classification has been greatly ad- 
vanced by their microscopic examination. The study of 
rocks was revolutionized by Sorby’s method of cutting trans- 
parent sections for microscopic study, and the same process 
applied to ores has often replaced speculation by direct 
evidence. The microscopic study of ores is hindered by the 
opacity of many of the species, but opaque minerals are 
examined in polished surfaces (for the method, see R. W. 
Van der Veen, Mineragraphy, The Hague, 1925). Micro- 
scopic study shows the conditions under which the ore was 
formed, the order in which the different constituents were