The Elements of economic geology

Bibliographic data

Monograph

Identifikator:: 1773832379

URN:: urn:nbn:de:zbw-retromon-172798

Document type:: Monograph

Author:: Gregory, John W. http://d-nb.info/gnd/11683014X

Title:: The Elements of economic geology

Place of publication:: London

Publisher:: Methuen

Year of publication:: 1928

Scope:: XIV, 312 S.; graph. Darst.

Digitisation:: 2021

Collection:: Economics Books

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Chapter

Document type:: Monograph

Structure type:: Chapter

Title:: Part I. Introduction

Collection:: Economics Books

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

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