6i2 FRANCIS PARKER SHEPARD 



recent world-wide movements which have removed the long accumu- 

 lated strains.^ The great glaciers which came down over Canada 

 and northern United States appear to have caused a decided depres- 

 sion of the land, which is shown by the great upHfts which have 

 occurred since the ice retreated.^ During the formation of geosyn- 

 clines there must have been subsidence at about the same rate at 

 which the sediment was added. This suggests that the burdening 

 due to the addition of the sediment was important enough to have 

 some effect on the deformation of the crust. Earthquakes are said 

 to be very common in the submarine portions of deltas offering 

 another indication of the weakness of the crust. Also if the crust 

 is actually in an isostatic condition in all places, it must be very 

 weak. Thus the plea for a crust strong enough to accumulate 

 strains through long ages and then give away with great momentum 

 is not supported by observations. 



It would be foolish to deny that the earth is rigid to stresses of 

 short duration, but when stresses are applied over fairly long periods 

 of time, thousands of years for example, the most rigid of bodies may 

 yield. Experiments on the strength of material may have sufi&cient 

 pressure to be compared to the forces acting beneath miles of rock, 

 but those experiments cannot be continued long enough to give any 

 idea of the effect of the time factor. 



Crustal creep toward mountain ranges. — ^A point which seems to 

 have been neglected by most geologists who beHeve in the shrinking 

 of the earth, is the means by which all of the shortening of the crust 

 is concentrated within a relatively small portion of the earth's sur- 

 face, namely the mountain ranges. If a balloon is inflated and cov- 

 ered with paraffin and some parts of this paraffin are thinner or 

 weaker than other parts, allowing some of the air to escape from the 

 interior of the balloon will cause crumpling to take place in the 

 weaker zones of the paraffin while the remainder of the paraffin 

 sHdes along the surface of the balloon toward these zones of crum- 

 pHng.3 The shrinkage of the earth with the resulting development 

 of plains and mountain ranges must be a process very similar to this. 



' T. C. Chamberlin, Jour. ofGeol., Vol. XXVII (1918), P- i97- 

 " R. A. Daly, Bull. Geol. Soc. America, Vol. XXXI (1920), p. 303. 

 3R. T. Chamberlin and F. P. Shepard, "Some Experiments in Folding," Jour. 

 ofGeol., Vol. XXXI (1923), pp. 495-96. 



