GEOPHYSICAL LABORATORY. 105 



The problem as a whole, although in some measure complicated, was 

 found capable of very definite laboratory solution. The extension of the 

 study to include the potassium-bearing nephelites has already been begun. 



(15) Mixtures of amorphous sulphur and selenium as immersion media for the deter- 



mination of high refractive indices with the microscope. H. E. Merwin and 

 E. S. Larsen. Am. Jour. Sci. (4), 34, 42-47. 1912. 



Fused mixtures of sulphur and selenium are glassy when cold. By stand- 

 ardizing the mixtures with respect to their refractive indices by measure- 

 ments on prisms molded into the angle between glass plates, they can be 

 used to match the refractive indices of suitable substances immersed in them 

 and studied under the microscope. A chart showing the refractive indices 

 of various mixtures for several wave-lengths has been prepared, and a 

 method of interpolating values of refractive indices, obtained by using a 

 monochromatic illuminator, explained. 



(16) Microscopical petrography from the quantitative viewpoint. Fred. Eugene 



Wright. Jour. Geol., 20, 481-501. 1912. (In press.) 



In this paper attention is directed to the importance of good quantitative 

 work in microscopical petrography which has now passed the qualitative, 

 reconnaissance stage of its development and is entering upon large problems, 

 essentially quantitative in nature, which require precise data of observation 

 for their solution. The different optical properties used in the determina- 

 tion of minerals are classified in detail and simple effective methods are 

 briefly described which experience has shown to be well adapted for the de- 

 termination of the different optical constants of mineral plates and grains. 



(17) Petrographic study of the specimens of loess, tierra cocida, and scoria collected 



by the Hrdlicka-Willis Expedition. Fred. Eugene Wright and Clarence N. 

 Fenner. Included in the volume Early man in South America, by Ales 

 Hrdlicka in collaboration with W. H. Holmes, Bailey Willis, Fred. Eugene 

 Wright, and Clarence N. Fenner. Bulletin No. 52, 55-98 (1912), Bureau of 

 American Ethnology, Smithsonian Institution. 



This collection was found to contain several extraordinary rock types, and 

 in approaching the problems which they present three distinct lines of at- 

 tack were followed : ( 1 ) the usual detailed petrographic-microscopic exam- 

 ination of the rocks; (2) chemical study of the different rock types; (3) 

 thermal study of the specimens at different temperatures and comparisons 

 of the products thus obtained with the natural products. 



The most important conclusions resulting from these three lines of inde- 

 pendent evidence are : 



(1) The loess consists in large measure of volcanic and eruptive material. 

 Salic volcanic glass is present in practically every specimen and may become 

 so abundant that it constitutes 90 per cent of the whole. The minerals are 

 remarkably fresh and unaltered, while the amount of argillaceous material 

 present is relatively small in most of the specimens. These facts may be con- 

 sidered indicative of tremendous and widespread volcanic activity of the 

 explosive type during or just preceding the formation of the loess. 



(2) The specimens of tierra cocida are composed, for the most part, 

 simply of loess fragments which have been indurated and reddened by heat 

 action, between 850 and 1050 . The loess and tierra cocida are similar in 

 general character and composition. 



(3) The scorice are not normal volcanic scoriae. They have been pro- 

 duced by the melting down of an original clastic material (loess) under con- 



