236 GEOLOGY. 



If the continental segments, taken as deep masses, weighed more than 

 the equivalent portions of the sub-oceanic segments, they would have 

 tended to settle down and force the latter up until they reached hydro- 

 static equilibrium. This would of course be opposed by the rigidity 

 of the rock, as in the previous case. If the continental segments had 

 a lower specific gravity than the oceanic segments, so that the two were 

 in essential balance, there would have been no tendency of the former 

 to sink, though the tendency to spread, as discussed above, would still 

 remain. 



Pendulum observations seem to show that, on the whole, the con- 

 tinental segments are now in approximate hydrostatic equilibrium 

 with the oceanic segments, and on this is built the doctrine of isostasy, 

 or the essentially balanced condition of the earth's crust, notwith- 

 standing the inequalities of its height. 1 At the same time, pendulum 

 observations seem to show that notably elevated regions, as for example 

 that of the Cordilleran tract, stand higher than they would if they 

 were in isostatic balance. 2 There is in such evidences, and in other 

 considerations that cannot be stated with sufficient brevity for this 

 place, ground for thinking that during profound bodily deformations 

 the continental segments may be pressed up, in portions at least, 

 beyond the plane of equilibrium, and that during the period of qui- 

 escence that follows, they tend to settle back to the equilibrium plane. 



If this view be well taken, there is ground for believing that some 

 portions of the Cambrian continents were pressed up beyond the plane 

 of isostatic equilibrium during the late Proterozoic deformations, and 

 that they slowly subsided during the long Cambrian period. In the 

 nature of the case, such a movement would be unsymmetrically dis- 

 tributed, and the result would be manifested in slow, quiet warping. 

 It would be more efficient in the early stages of the quiescent period 

 than later, for erosion would join with the subsidence to relieve the 

 overweighting. 



(c) Isostatic adjustments due to gradation. — If the continent had been 

 in isostatic equilibrium at the opening of the Cambrian period, the re- 



1 Dutton, Greater Problems of Physical Geology, Bull. Phil. Soc. Washington, 

 XI; also Am. Jour. Sci., Ill, 1874; Woodward, Mathematical Theories of the Earth, 

 Vice-Pres. Address, Math. Sect. Am. Assoc. Adv. Sci., 1899; also Smithsonian Rept. 

 for 1890. 



2 Putnam and Gilbert, Bull. Phil. Soc. Washington, Vol. XII, pp. 31-75, and 

 Gilbert, Jour, of Geol., Vol. Ill, p. 331, 



