TO QUESTION PERIODIC DIASTROPHISM 6ii 



explain at the same time the apparent periodicity of orogeny, if 

 there is such, and the periodic incursions of the sea. When the 

 ocean bottom was taking up the brunt of the shortening of the 

 crust, the sea-level would rise in relation to the lands. Then 

 when the chief orogenic disturbances were within the continents, 

 the bottom of the sea would be relatively depressed and the epi- 

 continental seas would be drained. 



THEORETICAL CONSIDERATIONS 



Is it not questionable whether the idea of periodic diastrophism 

 would ever have originated had it not been for some early field 

 evidence which suggested it ? If the interior of the earth is shrink- 

 ing constantly, whether at an increasing or a decreasing rate, the 

 non-shrinking outer crust might naturally be expected to fit itself 

 to the interior without interruption. The crust is variously com- 

 puted to support from g-^-g- to g-Q-Q- of its own weight so that it is 

 not reasonable to expect the rigidity of the crust to allow an open 

 space to develop underneath due to the contraction of the interior 

 without a wrinkling of the crust to fit it. 



If the crust could store up stresses for a long period before giving 

 away to them as suggested by Chamberlin,^ would it not be reason- 

 able to suppose that diastrophism would occur in only one zone 

 after the breaking point was reached ? Once one zone of weakness 

 collapsed, the pressure on other points, where relief was apt to take 

 place, would be reduced and a crowding toward the range where 

 deformation had set in would be expected, while quiet prevailed 

 elsewhere. This state of affairs is not what is looked for by the advo- 

 cates of periodic diastrophism, but on the contrary they speak of a 

 contemporaneous development of diastrophic movements in many 

 parts of the world. 



If the crust is rigid enough to store up stresses during long 

 periods, then burdening of the crust would not be expected to cause 

 yielding for long periods. The constant warping of the crust in 

 many parts of the world during the present cycle suggests that the 

 crust is rather easily disturbed from its state of equiHbrium. This 

 warping is especially significant if we agree with T. C. ChamberHn 

 in believing that the present isostatic condition of the crust is due to 



I T. C. Chamberlin, Jour, of Geol., Vol. XXVII (1918), P- i93- 



