400 EDWARD STEIDTMANN 



In the deep zones of high pressure and temperature, where 

 there is only slight mobility of the residual materials, this result 

 may be reached and perpetuated for a long time until, in the 

 course of geologic ages, they finally become the shallow zones of 

 low temperature and pressure. Here the residuals of metamor- 

 phic processes as well as the materials lost are forever in a state of 

 motion in response to the movements of the atmosphere and the 

 hydrosphere, controlled by gravity and the sun. Thus the prod- 

 ucts of metamorphism are redistributed into the sedimentary 

 rocks, and these in turn are reworked and redistributed. In this 

 redistribution lies the potentiality of an increase of the ratio of 

 calcium to magnesium of the lands with geologic time. 



Has sedimentation increased the ratio of calcium to magnesium 

 of the lands during geologic time? 



The sedimentary rocks are derived from other sediments and 

 from igneous rocks, ultimately they are derived from igneous 

 rocks. Clarke's average igneous rock is generally accepted ' as 

 representing the approximate composition of the primitive litho- 

 sphere. The criticism may be offered that this average is neces- 

 sarily not based on a study of the volumetric importance of the 

 various igneous rock types in the primitive lithosphere. It has 

 also been maintained that the igneous rocks themselves are very 

 largely derived from the fusion of sediments, which may be so 

 but has not been proven. The recurrence of certain predominant 

 igneous rock types at various times and places suggests that the 

 composition of magmas has not been materially influenced in the 

 way which one would expect from regional subfusion of sedi- 

 ments. It seems probable that the primitive lithosphere had a 

 composition between rhyolite and basalt, which is expressed in 

 Clarke's average. Approximations, not finalities, seem all that 

 can be hoped for in this problem. 



By an ingenious graphic method, W. J. Mead 1 estimates that 

 the average igneous rock is equivalent to shales, sandstones, and 

 limestones of Clarke's average compositions in the ratio of 80: n :g. 



F. W. Clarke 2 has made a similar estimate, based on average 



1 W. J. Mead, "Redistribution of the Elements in the Formation of Sedimentary 

 Rocks," Jour. Geol., XV (1906), 238. 



2 F. W. Clarke, "Data of Geochemistry," Bull. 330, U.S.G.S. (1908). 



