234 GEOLOGY. 



the crushing strength of some of the weaker rocks, and is a notable 

 percentage of the crushing strength of even the strongest ones. Under 

 "Jess pressure than this, rocks are often observed to creep, as in mines 

 and canyons. It is not improbable, therefore, that a pressure of this 

 magnitude, constantly exerted for a prolonged period, might cause 

 some spreading of the great continental platforms, and hence (1) some 

 lowering of their surfaces; and (2) at the same time some rise of the 

 sea-level. 



Such a force, perpetually in readiness to act, if not actually produc- 

 ing movement, would always cooperate with any other agency that 

 tended to- work in the same direction, or to develop weakness in the 

 resisting power of the continental mass. Cooperating with the molecu- 

 lar changes concerned in rock alteration, this pressure would tend to 

 give the rocks lateral extension in the line of least resistance. Whether 

 it is really very effective or not, it is at least a constant agency tending 

 to produce lateral creep, and hence to cause a lowering of the surface 

 of the continents, a rise of the surface of the sea, and therefore a spread- 

 ing of the ocean waters. 



We have considered this point on the basis of the present relative 

 heights of the continents above the ocean floor. A little considera- 

 tion will show that we are not likely to be far in error in applying simi- 

 lar figures to the Cambrian continents, for if the volume of the ocean 

 was then not far from what it is to-day, the ocean basins must have 

 had much the same capacity as now, and as the continental masses 

 appear to have had much the same area as they now have, the relief 

 in Cambrian times may be assumed to have been of the same general 

 order of magnitude as now. It may at least be assumed to have been 

 sufficiently great to have involved these considerations. It will be 

 seen that continental spreading or creeping will, within the limits of 

 its operation, diminish the capacity of the ocean basins, and so tend 

 to cause the waters to overflow the lands. 



No attempt is here made to definitely estimate the efficiency of 

 this agency, but some considerations which bear upon it may be cited: 

 (1') It cannot, probably, have been of a very high order, or it would 

 have affected adversely the perpetuation of the continents to a greater 

 degree than the record indicates. If the continental rock -masses 

 spread with anything like the facility that continental ice-masses do, 

 the land would probably have been completely submerged between 



