122 Adams and Coker — Elastic Constants of Rocks. 



of an entirely different class from the others, and furthermore 

 one which shows so much hysteresis that the application of 

 this method to it is less satisfactory than in the case of the 

 other rocks of the series. 



These results may be presented as follows : 



Average of D 



Marbles and limestones 6,345,000 



Granites ..:_.. 4,399,000 



Basic intrusives _.. 8,308,000 



The canse of the much greater compressibility of granite as 

 compared with the marbles and basic intrusives is not clear, 

 but would seem to be connected with the presence of quartz. 

 The only determination of the cubic compressibility of quartz, 

 so far as can be ascertained, is one by Voigt,* the ■ value 

 obtained being 5,504,190 pounds (387 X 10 6 grams per. sq. 

 cent.). This compressibility, as will be seen, is much greater 

 than that found in the case of either the limestones or the 

 basic intrusives, and while not in itself sufficiently great to 

 account for the high compressibility of the granites, goes to 

 show that in the quartz we have a mineral which is more com- 

 pressible than the ordinary rock-making minerals which form 

 the chief constituent in the rocks of the series examined. 



The marbles and the limestones of the earth's crust are con- 

 lined to its most superficial portion, resulting as they do from 

 the process of sedimentation. There is every reason to believe, 

 however, that what we may term the sub-structure of the 

 earth's crust is composed of acid and basic plutonic igneous 

 rocks. These make up the lowest part of the crust to which 

 we have access, and are found coming up from the still greater 

 depths. 



The cubic compressibility, D, of the earth's crust must lie 

 between the values given above for the granites and the basic 

 intrusives, approaching one or other of these values according 

 to the relative proportion in it of one or other of these classes 

 of rocks. 



If we take the average of the values obtained from these 

 two classes of rocks as represented by the seven granites and 

 the five basic intrusives (including the nepheline syenite), the 

 value obtained for D is 6,353,500. 



This, as will be seen, differs but little from the value of D 

 obtained for plate glass, which is 6,448,000. 



If, therefore, the earth's crust be composed of granite and 

 basic igneous rocks in approximately equal proportions, its 

 compressibility will be that of glass. If it be composed almost 

 exclusively of granite, the earth's crust will be more compres- 

 sible than glass ; and if the basic rocks preponderate very 

 largely, it will be less compressible than this substance. 



* Quoted in Becker, Experiments on Schistosity and Slaty Cleavage, 

 Bulletin 241, U. S. Geol. Survey, p. 32. 



