GEOPHYSICAL LABORATORY. 165 



heating or cooling, by E. D. Williamson and L. H. Adams, Phys. Rev., 14, 99-114 (1919); 

 The coohng of optical glass melts, by H. S. Roberts, J. Am. Ceram. Soc, 2, 543-56-3 (1919); 

 The relations between birefringence and stress in various types of glass, by L. H. Adams 

 and E. D. Williamson, J. Wash. Acad. Sci., 9, 609-623 (1919); Strains due to temperature 

 gradients, with special reference to optical glass, by E. D. Williamson, J. Wash. Acad. Sci., 

 9, 209-217 (1919) . In the present paper anneaUng of glass is treated as a whole. 



(397) The chemistrj' of the earth's crust. Henry S. Washington. J. FrankUnlnst., 190, 



757-815 (1920). (Reprinted, Smithsonian Misc. Coll., 1921.) 



After brief consideration of the interior of the earth, the general character 

 of igneous rocks is discussed, and the presence of water-vapor and other gases 

 in the magma is pointed out. In the discussion of the mineral character of 

 rocks, stress is laid on the fact that the number of essential rock-forming 

 minerals is very small. These are mostly silicates of aluminum, iron, mag- 

 nesium, calcium, sodium, and potassium. Any two or more of these minerals 

 (with two exceptions) may occur together and in all proportions. 



The chemical character of igneous rocks is summarized and the ranges 

 and maxima of the various constituents are given. The average igneous rock 

 is considered and, after some discussion of the sources of error involved in the 

 calculation, a new average (based on 5,159 analyses) is given. The average 

 rock is shown to be approximately a granodiorite. 



The average composition of the earth's crust in terms of elements is given. 

 Twelve elements (oxj^gen, sihcon, aluminum, iron, calcium, sodium, potas- 

 sium, magnesium, titanium, hydrogen, phosphorus, and manganese) make 

 up 99.61 per cent of the crust. 



The elements are referred to two main groups in the periodic table: (1) the 

 "petrogenic" elements, characteristic of and most abundant in igneous rocks, 

 of low atomic weight and occurring normally as oxides, sihcates, chlorides, 

 and fluorides; (2) the "metallogenic" elements, rare or absent in igneous rocks 

 but occurring as ores, of high atomic weight, and forming in nature metals, 

 sulphides, arsenides, etc., but not oxides or silicates. The suggestion is made 

 that beneath the silicate crust of petrogenic elements is a zone essentially of 

 nickel-iron, and beneath this a central core of the metallogenic elements. 

 This vertical distribution is in accord with Abbot's views as to the dis- 

 tribution of the elements in the sun. 



In igneous rocks and minerals the elements show a correlation, in that 

 certain of them are prone to occur with others, and a similar Umited correla- 

 tion is apparently true of the animal and vegetable kingdoms. 



The idea of "comagmatic regions," that is, the distribution of igneous rocks 

 in regions of chemically related magmas, is discussed and some are briefly 

 described. 



The calculation of rock densities from their chemical composition is dis- 

 cussed, and the average chemical compositions and densities of the conti- 

 nental masses and oceanic floors are given. It is shown by these that the 

 average densities of the continents, ocean floors, and various smaller regions of 

 the earth stand in inverse relation to their elevations. The bearing of this 

 relation of average density and elevation on the theory of isostasy is pointed 

 out, and it is shown that the data presented are confirmative of the theory. 



(398) The system cupric oxide, cuprous oxide, oxygen. F. Hastings Smyth and H. S. 



Roberts. J. Am. Chem. Soc, 42, 2582-2607 (1920). 



It has been shown that solid solution of cuprous oxide in cupric oxide does 

 not take place in the temperature range where both oxides remain soUd. 

 Previous results indicating such solution may probably be explained by lack 

 of careful temperature control, and by possible adsorption of nitrogen in solid 



