196 SINGLES THOM SIA CHDIEIN TES 
fragments may be rounded, and such rocks resemble ordinary 
clastic rocks. In this case the rock becomes a pseudo-conglom- 
erate, and there are all gradations between such a rock and one 
in which the cracks and crevices become subordinate, the defor- 
mation being chiefly that of flowage. For a soft shale, but a 
small thickness of superincumbent strata, possibly 500 meters or 
less, may prevent any considerable fractures and crevices from 
forming. For the strongest massive rocks, a great thickness of 
superincumbent strata, possibly nearly 10,000 meters, may be 
necessary to prevent cracks and crevices from forming. 
Heim states that it is impossible for crevices and cracks to 
exist at so great a depth as 5000 meters.t. From geological 
observations I have for some time been convinced that a greater 
depth than this is required to close crevices under some condi- 
tions. The depth at which a cavity of any definite size begins 
to close will depend upon its form, upon the strength of the rock, 
upon whether the rock is saturated with water, upon the increased 
plasticity of the rock due to the rise of temperature with 
increased depth, upon the amount of lateral thrust, and upon 
the length of time during which the rock is subjected to stress. 
So large an opening as the St. Gothard tunnel, 6.4 meters 
high and 8 meters wide, exists with no observed tendency to 
close under an irregular dome of rock which for some distance is 
more than a mile thick and has a maximum thickness of 1830 
meters. The thickness of the superincumbent rock diminishes 
from the maximum to nothing at the ends of the tunnel. How- 
ever, it would appear from this case to be highly probable that 
in order to close cavities the maximum number must be multi- 
plied by a factor of considerable magnitude. 
It would seem that mathematicians and physicists, after exper- 
iments upon the plasticity of rocks of different kinds, ought to 
give an approximate solution of the problem as to the depth at 
which cavities in any kind of rock would begin to close. We 
need long continued experiments upon the plasticity of different 
rocks at various temperatures and pressures, and while the rocks 
* Mechanismus der Gebirgsbildungen, by ALBERT HEIM, Band II., 1878, p. 110. 
