GEOPHYSICAL LABORATORY. 147 



(460) Isostasy and rock density. Henry S. Washington. Bull. Geol. Soc. Amer., 33, 

 375-410. 1922. 



In this paper, which is an expansion of part of a former one ("The chemistry 

 of the earth's crust," by Henry S. Washington, pubhshed in the Journal of the 

 FrankKn Institute, vol. 190, pp. 757-815, 1920, reviewed in Year Book No. 

 20, p. 165), there is presented a study of the relations between the average 

 densities of different areas of igneous rocks and their average altitudes, with 

 the object of correlating these relations with the theory of isostasy. The out- 

 come of the discussion is that the general relation holds good: the average 

 density of the igneous rocks varies in the opposite sense as the average altitude. 

 The results of the study, therefore, harmonize with and corroborate the 

 theory of isostasy. 



Comagmatic regions, in which the igneous rocks are shown by their similar 

 chemical and mineral characters to be derived from the same magma, are 

 briefly discussed, especially as to their bearing on isostasy, and the distribu- 

 tion of them throughout the United States is described. 



The normative method for arriving at the average density of a region is 

 described in detail. This method consists in calculating the norm of the 

 average rock of the region and, from the known densities of the normative 

 minerals, calculating the average density. The average densities of many 

 areas over the globe, including the earth as a whole, the continents and ocean 

 floors, and many countries and states, resulting from these calculations, are 

 given in a table and their relations to the corresponding altitudes are dis- 

 cussed. 



The physical method of calculating the average density, which consists in 

 averaging the specific gravities determined by the balance, is discussed and 

 some results are given. It is shown that this method is not capable of yielding 

 such consistent and reliable results for the present purpose as the normative 

 method. 



The depth of the "isopiestic level" or "depth of compensation," at which 

 all the various columns of inversely varying densities and heights balance 

 each other or exert the same pressure, is discussed. The depth is calculated 

 from the known average altitudes and the average densities calculated by the 

 normative method, using different sets of areas. It is shown that the norma- 

 tive method is capable of yielding reliable results with proper data, and that 

 the most probable value on this basis for the isopiestic depth is 59 km., which 

 agrees well with Bowie's value of 60 km., arrived at by gravity methods at 

 stations distributed over the United States, but which differs from that of 

 96 km. derived from mountain stations and accepted by Bowie as the best. 

 It is shown that the former value is probably the more correct. 



The correspondence between the distribution of comagmatic regions and 

 areas of positive and negative anomaly in the United Sates is briefly dis- 

 cussed. 



(461) The system FejOs-SOs -H2O. E. Posnjak and H. E. Merwin. J. Am. Chem. Soc, 

 44, 1965-1994. 1922. 



The system Fe2O3.SO3.H2O has been studied over the temperature range 

 from 50° to 200°. The following crystalline phases were encountered: Fe203; 

 FeaOa.HaO; 3Fe2O3.4SO3.9H2O; Fe2O3.2SO3.H2O; Fe2O3.2SO3.5H2O; 2Fe203. 

 6SO3.I7H2O; Fe203.S03 (2 forms); Fe2O3.3SO3.6H2O; Fe2O3.3SO3.7H2O; 

 Fe2O3.4SO3.3H2O; and Fe2O3.4SO3.9H2O. 



The conditions under which these substances are formed, the range of their 

 stability, and their relation to one another, also some of their crystallographic 

 and optical properties, were determined. The data obtained are presented 

 by means of tables, and graphically by means of curves and a solid model. 



