138 CARNEGIE INSTITUTION OF WASHINGTON. 



constituent minerals, provided that the rock is holocrystalHne. In general, 

 the compressibility decreases with increasing basicity, that is, with decreasing 

 silica-content. Quartz and metallic iron may be considered to stand at 

 the two extremes as regards the compressibility of igneous rocks. In any 

 given class of rocks the variation in compressibility at high pressures is small 

 and almost wholly due to the variation in composition; at low pressures 

 the variation from one rock to another in the same class is much larger and 

 depends mainly on the degree of compactness of the rock. The granites as 

 a rule show at low pressures a compressibility much above what may be 

 called the normal compressibility, but basic rocks may preserve their rela- 

 tively small normal compressibility down to low pressures. At 10,000 

 megabars the compressibility of a typical granite is about 1.9X10"^ per 

 megabar and that of a typical gabbro is about 1.2 XIO"^ 



From the ratio of the velocities of the two kinds of waves transmitted 

 through the earth and from the known elastic behavior of compact rocks, 

 there is reason to believe that the value of Poisson's ratio for rocks at moderate 

 or high pressures is very close to 0.27. From this value and from the meas- 

 ured compressibility the rigidity of a number of typical rocks is calculated. 

 The rigidity of a typical granite at 10,000 megabars is 0.3X10^ megabars; 

 that of a typical gabbro is 0.5X10^; and that of an ultra-basic rock, e. g., 

 a dunite, is 0.6 XIO^ The rigidity of the earth as a whole, which is prob- 

 ably near that of steel (0.9X10^), is higher than the value for the most 

 rigid silicate rock; but doubtless the rigidity of a basic or ultra-basic rock is 

 raised enough by a pressure corresponding to a few hundred miles of rock 

 to yield the required value. 



From the values of the bulk modulus (the reciprocal of the compressibility) 

 and of the rigidity the velocities of the two kinds of waves transmitted through 

 the earth are calculated for the various types of rocks. With increasing 

 basicity of the rocks the velocity of the longitudinal vibrations increases 

 steadily from the value for granite, 5.6 km./sec, to that for dunite, 7.4 

 km. /sec, but is lower, 6.0 km./sec, for metallic iron. The initial velocity of 

 the longitudinal vibrations is usually taken to be a little above 7 km./sec. 

 This corresponds to a gabbro or to a pyroxenite, and furnishes an indication 

 of basic or ultra-basic material at a relatively small depth below the surface of 

 the earth. 



The bearing of the results for the elastic constants of rocks on the nature 

 of the earth's interior will be discussed further in a subsequent paper. 



(485) A chemical study of the fumaroles of the Katmai region. E. T. Allen and E. G. 



Zies. National Geographic Soc, Contributed Technical Papers, Katmai 

 Series, No. 2. 1923. pp. 75-155. 



The material of this publication has been summarized in the annual report 

 for 1922, Carnegie Institution Year Book 21, pp. 128-130. 



(486) The geographical study of population groups. M. Aurousseau. Geographical Re- 



view, 13, 266-282. 1923. 



The different kinds of population groups are briefly described, and the trend 

 of their evolution into the groups of the present day is discussed. The 

 principle is deduced that the character and development of the group are the 

 result of interaction between the group and its region. The historical side 

 of the growth of the group in its region is examined, and it is shown that 

 overpopulation of the region tends to adjust itself by revolutionary change in 

 the method of occupation. The supplanting of ruralism by urbanism at the 

 present time, and the accompanying evolution of the region of simple and 

 uniform production, is beheved to have assisted the rise of the conurbations. 

 These in turn have made the world as a whole their region of supply, and 



