CONDITION OF THE INTEKIOR OF THE EAETH. 495 



subject to such pressure that their yiekliug uuder shearing straius exceeding the 

 elastic limit is not by fracture, but by flow. I conceive the orogenic blocks as con- 

 fluent with the subjacent layer, excepting such as may wedge out by the convergence 

 of fault planes.' 



This view is closely allied with that which regards the interior as solid, 

 and, indeed, if I understand the authors holding the doctrine of solidity, 

 forms a necessary postulate of their explanation of orogenic and epeiro- 

 genic movements. It is again well stated by Becker, who regards the earth 

 "as a solid mass of extremely high viscosity which would yield slowly to 

 relatively moderate forces of constant terrestrial direction and long dura- 

 tion, but which would probably yield almost imperceptibly to any force 

 of brief duration or rapidly changing direction."" For such a condition, 

 however, which seems to me probably or possibly true for all the earth 

 excepting its volcanic areas, I should prefer,, as more intelligible to ordi- 

 nary readers, to speak of the interior as plastic rather than as either solid 

 or liquid, though in its rigidity or resistance to change of form it may 

 equal or surpass the hardest rocks of the earth's surface. 



In the present state of our knowledge, the elevation of the area of 

 Lake Agassiz, increasing in amount from south to north, during the depart- 

 ure of the ice-sheet, seems most clearly intelligible by supposing it to have 

 been an uplifting of the crust by the inflow of plastic if not perfectly 

 molten rock from districts outside the glaciated area, occumng probably 

 between the depths of 20 and 100 miles, in obedience to gravitation, 

 which, to preserve the condition of isostasy, would cause the crust, when 

 loaded by the ice-sheet, to sink and displace part of the plastic interior, and 

 when the ice-sheet was removed would cause the })lastic rock to flow back 

 and raise the crust approximately to its former height. 



It must be confessed that we have only a very inadequate knowledge 

 of the conditions which Avould result from the enormous pressure and high 

 temperature of the earth's interior; and wide diversity in speculations on 

 this subject will probably long continue. Professor Shaler, while holding 

 that the earth is niainly solid throughout, perhaps having in its most mobile 

 layer beneath the crust "a rigidity such as belongs to the metals of average 



' U. S. Geol. Survey, Monograph I, Lake Bonneville, 1890, pp. 358, 359. 

 « BuUetiu, G. S. A., Vol. II, 1891, p. 70. 



