abstracts: geochemistry, petrology 83 



determined precisely as above. The difference between the calculated 

 amount of barium sulphate and that actually found is the correction to 

 be applied to the weight of the precipitate obtained in the actual analysis. 

 This procedure, as compared with that advocated by Allen and John- 

 ston, is easier and much more rapid; it is, however, not so generally 

 applicable, but may be used whenever the composition of the solution 

 containing the sulphate to be determined is known approximately; and, 

 we believe, will yield results, accurate to ± 0.05 per cent of the total 

 sulphate present, ir. most cases likely to occur in general analytical work. 



J. J. and L. H. A. 



GEOCHEMISTRY.— The data of geochemistry (second edition). F. 



W. Clarke. Bulletin U. S. Geological Survey No. 491. Pp.782. 



1911. 

 This work was first published in 1908. The present volume has been 

 revised and much enlarged. F. W. C. 



PETROLOGY. — The methods of petrographic-microscopic research: 



Their accuracy and range of application. Fred. Eugene Wright. 



Publication No. 158, Carnegie Institution of Washington. (In 



press.) 



During the past six years the work with artificial silicate preparations 



in the Geophysical Laboratory has imposed new and difficult problems 



to be solved by the microscope. Not only are such preparations very 



fine-grained, but the degree of accuracy of each measurement must be 



definitely known if it is to be applied without reserve to geophysical 



problems. To meet these new conditions, it has been necessary to devise 



new methods, involving extensive alterations in the microscope, and 



also to test the different methods available for the determination of the 



optical constants of minerals in the thin section and to ascertain their 



relative accuracy and general applicability. As a result of these tests, 



the methods best adapted for work with artificial and all fine-grained 



preparations are now fairly well established and their application has 



become in large measure a matter of routine. 



The present publication aims to offer a connected presentation of the 

 entire investigation of the petrographic microscope so far as it has been 

 carried, in which the different methods are coordinated and the signifi- 

 cance and usefulness of each particular method is made to appear in its 

 proper relation. An effort has been made to establish the limits of accur- 

 acy of each method as ordinarily used, and also the limits of accuracy 

 theoretically attainable in measurements of this kind. 



