812 



DYNAMICAL GEOLOGY. 



The expansive force of moisture gaining access to such beds has been appealed to as 

 a cause of elevation and fiexures. But such sedimentary beds contain their maximum 

 of moisture when formed ; and with this, although situated at the bottom of the 

 trough, they have still laid quietly as to any folding (not, it may be, as to consolida- 

 tion) until the epoch of mountain-making has finally come; and in the mean time the 

 effect from any expansion did not prevent progress in the subsidence. 



With a rigid globe, the source and extent of volcanic and other igneous action are 

 insufficiently explained. For the sake of an explanation, the solid interior has been 

 assumed to be just at the verge of fusion ; and areas of liquidity have been supposed 

 to come from a sudden diminution of pressure, on the principle that pressure modifies 

 the fusing limit (p. 809). But how such a diminution could have been produced for 

 the multitudes of cases of igneous eruptions over the globe, along lines often a thousand 

 miles or more in extent, is not satisfactorily explained. It has been supposed that 

 mountain lifting may sometimes make open spaces below ; but no method in a rigid 

 globe could do this, unless expanding vapors were indefinite in supply for the purpose, 

 and hardly then ; and open spaces so made could not remain such. Erosion over the 

 surface has been urged as a cause, but this, at its most rapid rate, tends only to lower 

 the isogeothermal planes just as the accumulation of beds raises them (p. 718). 



Further, the great faults in the rocks have no adequate explanation 

 on the view of a very thick rigid crust or a rigid globe ; for there 

 could be no faulting in such a globe below the limit of the supercrust, 

 unless from the improbable condition of an unequal heating of the 

 two sides ; and there could be likewise no friction or crushing to make 

 the heat of fusion. 



Among geological facts, none appears to demand for its explana- 

 tion a rigid globe. The demand has come through the supposed re- 

 quirements of physical laws, studied with the aid of the highest math- 

 ematics, whose methods and conclusions are sure only when all the 

 modifying conditions of the problem are thoroughly understood. It 

 is now admitted by some of the best of physicists that no arguments 

 have yet been presented which prove the earth to be a rigid globe, or 

 to have a rigid crust a thousand miles or so thick ; and it is also ad- 

 mitted by some mathematicians and physicists of eminence, including 

 Airy, the Astronomer Royal, that the hypothesis of a thin crust over 

 a liquid interior is probably the true one. 



The Science of Geology is, therefore, free to adopt the conclusion 

 which seems best to suit known facts. 



2. The Force engaged in Mountain-making. 



The present actual thickness of the earth's crust, or that at any par- 

 ticular period in the past, may always remain unknown. All that geol- 

 ogy can claim to teach is that it is thin enough to undergo some change 

 of level through the forces within or beneath it. That the reader 

 may better appreciate the problem, a cut is here introduced (Fig. 

 1159) representing a section of a hemisphere, supposing the crust onl} 

 one hundred miles thick — that is, a fortieth of the radius. In the cut 



