RECENT ADVANCES IN SCIENCE 30* 



and some 1,200 square miles of available coal. The Swaziland 

 coal is partly semi-bituminous, and partly excellent anthracite. 



In the same publication A. E. V. Zealley describes the 

 numerous chromite deposits of Selukwe, Rhodesia. These are 

 found in talcose schist and silicified serpentine. In spite of the 

 alteration of the enclosing rocks and the evidence of secondary 

 deposition of at least some of the chromite, Mr. Zealley is 

 inclined to ascribe the origin of the ore-bodies to magmatic 

 differentiation of chromite from an original ultrabasic magma. 



The relation of ore-deposits to igneous rocks receives con- 

 sideration in two papers in Economic Geology, Feb. — Mar., 

 191 5. From the petrological side L. V. Pirsson discusses the 

 origin of the copper, lead, and zinc ores, found in regions of 

 practically level sedimentary rocks, such as the State of Mis- 

 souri. Geologists have been reluctant to accept an igneous 

 origin for these ores on account of their remoteness from 

 any superficial signs of igneous activity. Prof. Pirsson holds 

 that they may have been deposited from the volatile con- 

 stituents of igneous magmas, which, instead of explosively 

 making their way to the surface as a volcano, have worked 

 their way quietly upward through the strata, and have been 

 largely absorbed by meteoric waters. 



B. S. Butler discusses the relations of ore deposits to different 

 types of intrusive bodies in Utah. The intrusions consist of 

 laccolites and stocks, of which the latter are subdivided into 

 those which have been truncated by the present plane of 

 denudation near their apex, and those truncated at greater 

 depth. The ore deposits associated with the apically-truncated 

 stocks are of much greater value than those associated with 

 the laccolites and deeply-truncated stocks. This fact is ex- 

 plained by the process of differentiation having concentrated 

 the lighter mobile and volatile constituents of the magmas, 

 which carry the metals and sulphur in solution, near the apices 

 of stocks, thus forming valuable metalliferous deposits. In 

 the deeply-truncated stocks these deposits have been eroded 

 away ; whilst in the laccolites the amount of magmatic material 

 was too small, and the differentiation too incomplete, to furnish 

 large ore deposits. 



The Geological Survey of the United Kingdom has pub- 

 lished a Handbook to the Collection of Kaolin, China-clay, and 

 China-stone in the Museum of Practical Geology, by J. A. 

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