98 FRANK D. ADAMS 
of Flow for very resistant rocks, such as granites, 1s 2,200 to 2,600 
meters or about a mile and a half below the surface of the earth, 
and considerably nearer the surface for limestone and other softer 
rocks. After thirty years of additional study Heim, in a recent 
paper, records his opinion that these depths are too small, that the 
Zone of Flow lies deeper within the earth’s crust, but as to how 
much deeper he does not venture an opinion.* 
President Van Hise in the interpretation of the results of his 
classic work on the ancient crystalline rocks of the United States 
in the district of the Great Lakes, reached a similar conclusion 
with reference to the twofold subdivision of the earth’s crust, but 
placed the upper surface of the Zone of Flow at a considerably 
greater depth than Heim, namely, 12,000 meters or 7.4 miles. 
Van Hise based his estimate on a mathematical calculation 
having as its starting-point the crushing weight of a cube of granite 
at the surface of the earth as determined by a testing machine in 
the ordinary manner adopted in testing the strength of building 
materials—granite being one of the strongest and at the same time 
one of the commonest rocks in the earth’s crust. This calculation 
was made for Van Hise by Professor Hoskins who, taking the 
figures for the crushing strength of granite thus obtained, endeav- 
ored to calculate the depth below the earth’s surface at which the 
pressure would be so great that all empty cavities would close as 
a result of plastic flow, even in the case of the hardest rocks like 
granite. This depth he fixed at four miles, or 6,520 meters. This 
is indicated by line “a” in Fig. 1. If however the cavities were 
filled with water Hoskins calculated that they would remain open 
to a depth of 6.4 miles, or 10,350 meters. This depth is shown by 
the line “6” in Fig. rc. 
Van Hise then assumed an additional factor of safety, and took 
12,000 meters as a depth at which not only all cavities would close 
but the hardest and most resistant rocks would flow—this being 
therefore the upper surface of the Zone of Flow in the earth’s crust. 
This depth has been indicated by the line “c”’ in Fig. 1. 
In order to make such a calculation, even in the very simple 
™ Albert Heim, Geologische Nachlese, No. 19 (Vierteljahrschrift der Naturfor. Gesel. 
in Ziirich, 1908, 45). 
