GEOPHYSICAL LABORATORY.* 



Arthur L. Day, Director. 



The present year has witnessed the close of two investigations of con- 

 siderable magnitude, both of which are intimately associated with nearly 

 every problem hitherto studied in the laboratory. 



The purpose of the firstf was to place the methods for the study of min- 

 erals with the petrographic microscope upon a quantitative basis, so far as 

 practicable. If we would study these matters precisely, we must have not 

 only an indication of what happens when minerals are brought into combi- 

 nation, but a definite numerical measure of the properties which are charac- 

 teristic of the resulting substance and which serve to distinguish it from the 

 initial ingredients. 



The purpose of the secondj was to perfect the apparatus and methods for 

 the accurate measurement of those extreme temperatures where the minerals 

 enter into combination. Supposing the chemical composition to be known, 

 two conditions suffice to define and to reproduce all that occurs when two 

 minerals are brought together — the pressure and the temperature. Methods 

 for measuring extreme temperatures with the highest accuracy are now pro- 

 vided ; of the measurement of pressure more will be said farther on. 



The Use of the Microscope. — It happens that the work with chemically 

 pure silicate preparations (mineral types) has imposed new and difficult 

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

 very fine-grained, but the quantitative character of the studies now being 

 undertaken imposes wholly new standards of accuracy for each measure- 

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

 methods involving extensive alterations in the microscope, and also to test 

 the existing methods for the determination of the optical constants of min- 

 erals in thin sections. The effort has been made to establish the limits of 

 accuracy of each method and also the limits of accuracy theoretically attain- 

 able in measurements of this kind. As a general result, it may be stated that 

 on clear individual grains measuring from o.oi to 0.03 mm. in diameter, all 

 the optic properties ordinarily studied in the microscopic investigation of 

 minerals in the thin section can now be determined with a satisfactory degree 

 of accuracy. This investigation, which has been of the most thorough-going 

 character, has occupied six years, and its results are now available to all stu- 

 dents of petrology. Some of the methods have already been adopted abroad 

 and are finding a wide field of usefulness. 



* Situated in Washington, D. C. Grant No. 744. $75,000 for investigations and 

 maintenance during 1912. (For previous reports see Year Books Nos. 3-10.) 



t Publication of the Carnegie Institution of Washington No. 158. 



t Publication of the Carnegie Institution of Washington No. 157 and Am. J. Sci. 

 (4), 33, 517-533. 1912. 

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