abstracts: petrography 115 



The most interesting result for geochemistry which was obtained in 

 these synthetic studies was the following: The unstable crystalline 

 forms, metacinnabar, wurtzite, and marcasite, are obtained only from 

 acid solutions, while the corresponding stable forms, cinnabar, sphaler- 

 ite, and pyrite, are the only product of alkaline solutions, tho they 

 may be obtained from acid solutions also. E. T. A and J. L. C. 



PETROGRAPHY. — Microscopical petrography from the quantitative view- 

 point. Fred. Eugene Wright. Journal of Geology, 20: 481- 

 501. 1912. 

 In this paper attention is directed to the importance of good quantita- 

 tive work in microscopical petrography which has now passed the quali- 

 tative, reconnaissance stage of its development and is entering upon 

 large problems, essentially quantitative in nature, which require pre- 

 cise data of observation for their solution. The different optical prop- 

 erties used in the determination of minerals are classified in detail and 

 simple effective methods are briefly described which experience has 

 shown to be well adapted for the determination of the different optical 

 constants of mineral plates and grains. F. E. W. 



PETROGRAPHY.- — 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 vol- 

 ume, Early man in South America, by Ales Hrdlicka in collaboration 

 with W. H. Holmes, Bailey Willis, Fred. Eugene Wright, and Clar- 

 ence N. Fenner. Bulletin, Bureau of American Ethnology, Smith- 

 sonian Institution, 52: 55-98. 1912. 

 This collection was found to contain several extraordinary rock types, 

 and for the solution of the problems which they present, three distinct 

 lines of attack were followed: (1) the usual detailed petrographic-micro- 

 scopic examination 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 pro- 

 ducts. 



The loess consists in large measure of volcanic and eruptive mate- 

 rial. 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 spec- 

 imens. These facts may be. considered indicative of tremendous and 



