68 L. D. LEET AND W. M. EWING 
ported behavior of granite stands out as an exception to the law demon- 
strated by Adams and Williamson “that at moderately high pressures the 
compressibility of a fresh holocrystalline rock is an additive function of the 
compressibility of its minerals”, and resulted in their modifying it to apply 
only “provided the pressure is not too low”. The following quotations sum- 
marize the evidence which Adams and Williamson had for this change and 
show the difficulty which they encountered in explaining it. 
In the discussion on page 523 of their report, accompanying the graph 
shown in Fig. 6 here, is the following statement: “The results of Adams and 
Coker for six different granites provide the basis for the estimate of the range 
SE 
= 
a= 
iv 
SSS 
LIZ LLL 
LLL, 
LLLLILLLILLLLUL LLL ULL AAU deb VZZZZZLLA 
<s 
Yy 
YY ie eee 
LOTTE. 
WSs 
LMM LLL EETXLTEETEEELEEEEEEEEETTEEZTEEEEE 
aS ESran) Fz 2S sed ee ee] ees eee eee 
ALOR ORIEL EOLIUIE A ESIEEMILEIL EET EEE EIDE TEEPE EEE, 
Peridotites 
Gabbros 
Compressibility, 8 X 10° 
LS 
y | 

0 2000 4000 6000 8000 10000 12000. 
Pressure in Megabars 
Fig. 6. Compressibility as a function of pressure, according to Adams and 
Williamson. The circle and dotted line have been added to their graph. 
of variation for granites, at low pressures. The corresponding variation for 
the gabbros at low pressures is admittedly a very rough estimate representing 
mainly the opinion of the authors.” 
Thus, the only evidence for the anomalous behavior of the compressi- 
bility of rocks at low pressures seems to be the results of Adams and Coker 
on six granites. Even a futile attempt to find a consistent explanation for the 
anomaly apparently did not lead Adams and Williamson to doubt the validity 
of the earlier results as average figures for low pressures. Porosity was looked 
upon as a possible cause, but after considering it at some length, they con- 
cluded: “None of the rocks has enough porosity to explain, on the basis of 
detached voids or channels, more than a few percent of increase of com- 
pressibility. ... The granites, moreover, although containing less than one 
212 
