CLASTIC CONSTANTS OF ROCKS. 67 
If the nepheline syenite be included with the basic rocks, an average value 
of D is obtained of 8,308,000. 
This omits from consideration the sandstone, it being a rock 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 limestone 6,345,000 
Granites 4,399,000 
Basic intrusives 8,308,000 
The cause 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 compress- 
ibility of quartz, so far as can be ascertained, is one by Voigt,* the value 
obtained being 5,504,190 pounds (387 x io 6 grams per sq. cm.). This com- 
pressibility, 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 suf- 
ficiently great to account" for the high compressibility of the granites, goes to 
show that in the quartz we have a mineral which is more compressible than 
the ordinary rock making minerals which form the chief constituents in the 
rocks of the series examined. 
The marbles and the limestones of the earth's crust are confined 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 values obtained for D of 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 
*Quoted in Becker: Experiments on Schistosity and Slaty Cleavage, Bulletin 241, 
U. S. Geol. Survey, p. 32. 
