DEPTH OF ZONE OF FLOW IN THE EARTH’S CRUST tor 
We find that at a depth of about six miles beneath the surface the pressure 
must become so great that all rocks known to us would be crushed by it. If 
it were attempted, for example, to tunnel in rock at this depth, the roof of the 
tunnel would immediately collapse and the opening be entirely sealed up. 
The microscopic pores in the rock would likewise and for the same reason be 
closed. 
As a matter of fact the figures for the depth of the Zone of Flow, 
which are under discussion, hang upon a slender thread of doubt- 
ful mathematical analysis enfolded by a cloak of many conjectures. 
In his Presidential Address before Section G of the British 
Association for the Advancement of Science in 1904, Hon. Chas. 
A. Parsons discusses among other questions the possibility of sink- 
ing a shaft into the earth’s crust to a depth of 12 miles, and ina 
letter which appeared in Nature (October 20, 1904) Geoffrey Martin 
expressed the opinion that at this depth the pressure would be so 
great that the walls of such a shaft, if it were constructed, would 
close in, owing to the viscous flow of the rock through which it 
passed. Parsons, in a note commenting on this letter, shows that 
a misplaced decimal point made the figures given by Professor 
Martin too high, and states that at the depth mentioned the 
pressure of the wall rock would amount to 44 tons per square inch, 
which in his opinion would be insufficient to close the shaft, adding: 
I think that the evidence at present available leads to the conclusion that 
after a small amount of shrinkage of the shaft sides inward had taken place, 
a state of equilibrium would be established, enabling the surrounding rock in 
its state of great compression to withstand the so-called hydraulic pressure 
down to a depth from the surface of at least 12 miles. 
He concludes, 
It would however be interesting to subject a cylinder of granite or quartz 
rock carefully fitted into a steel mould and having a small hole bored through 
its center to a pressure of say too tons to the square inch and see what shrink- 
age in the hole would result. 
As it seemed that important information might be obtained 
along the line suggested by Parsons, an experimental study of the 
question was undertaken, from which some rather interesting 
results have been obtained. 
To go back for a moment to the important work of President 
Van Hise, it will be noted that in looking over the factors enumer- 
