DEFORMATION OF ROCKS 201 
such temperatures and pressures even brittle rocks, under these 
great and very slowing acting forces, when saturated with super- 
heated water, obey the laws of hydrostatics, for plastic solids 
when strained beyond the limit of elasticity follow the same 
laws of deformation as do liquids. It is probable that the above 
considerations should reduce the estimate for the closing of 
cavities in the strongest rocks to 10,000 meters or less. 
It therefore appears highly probable that at a depth of 10,000 
meters not only do no crevices permanently exist in the earth, but the 
rocks are in such a condition that actual welding of the fractured parts 
would soon take place, supposing fracture to occur. 
Such was apparently the case in the deepest lying gneisses 
and anorthosites of the original Laurentian area, described by 
Adams.t Here in certain areas each of the individual mineral 
particles is broken into many fragments. No extraneous or 
infiltrated material is discovered between the granules, and yet 
the rocks are exceedingly strong and tough, showing in all 
probability that the broken particles were welded. However, 
there may possibly be a zone in which the deformation occurs 
by minute fractures of the individual particles, these being held 
together without interspaces, and yet where the temperature and 
pressure are not sufficient to cause welding. 
The conclusions as to the depth at which cavities close accord 
well with observations in the field and with the microscope. It 
is only in material from the cores of great mountain masses or 
in regions subjected to vast denudation that the microscope is 
unable to discover crevices caused by great deformation. The 
large secondary cracks and crevices which may have formed 
during the time when the rocks were nearing the surface by 
denudation are not here referred to, but the innumerable minute 
crevices affecting the individual grains, which were plainly pro- 
duced by the deep-seated deformation of the rock. In many 
instances in which such crevices are found it appears probable 
from the geology of the region that the rocks were buried under 
14 Further Contribution to our Knowledge of the Laurentian, by FRANK D. 
Apams. Am. Jour. Sci. (3), Vol. L., 1895, pp. 62, 63. 
