JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. IV SEPTEMBER 19, 1914 No. 16 



GEOPHYSICS. — Note on mean density of fractured rocks. George 

 F. Becker, Geological Survey. 



In connection with the theory of isostasy changes in the mean 

 density of rocks deserve study. Charles Babbage long since 

 called attention to the effects of temperature in this regard. 

 Another source of change in density is rupture; for since there is 

 every reason to believe that the elasticity of volume for crystal- 

 line solids is perfect, fissures in rocks represent voids, and dimin- 

 ish density so long as they remain unfilled by secondary dep- 

 osition. 



It is a familiar fact that orogenic movements are accompanied 

 by extensive and minute fissuring as well as by folding and dis- 

 tortion; and it is well known also that in many instances what 

 at first sight resembles plastic folding or contortion turns out 

 on close inspection to have resulted from the crushing of a bed 

 or layer under ' confinement. Many years ago I came to the 

 conclusion that the residual, unfissured fragments in the Coast 

 Ranges of California do not average larger than a hen's egg. 

 Such crushing must of course convert a vast amount of energy 

 into heat, and in my opinion Mallet's theories on this subject 

 have not received the attention they deserve. Thermal effects, 

 however, will not be discussed here, but only mechanical ones. 



A rude approxunation to what happens when a brittle bed 

 under confinement is bent may be imagined. The first step will 

 be the formation of not more than four systems of fissures divid- 



429 



