TO QUESTION PERIODIC DIASTROPHISM 607 



occurred on the continents and little of any importance on the sea 

 bottom. Estimates of the amount of shrinkage of the globe are 

 made by computing the amount of folding in the ranges on the lands 

 and allowing nothing for crustal shortening beneath the seas. If 

 the earth's interior is shrinking and the outside crust fitting itself 

 to a smaller interior, unless the ocean bottoms show no evidence of 

 past diastrophism, it would seem totally unreasonable to suppose 

 that all of the shortening occurred within the lands which form such 

 a small percentage of the total surface. 



That diastrophism is going on today beneath the ocean basins 

 is suggested by the numerous active volcanoes which occur within 

 the confines of these basins. The location of the epi-centers of 

 earthquakes shows more disturbance on the ocean bottoms than 

 within the lands. To be sure, some of these earthquakes may be 

 due to subsidence because of the weighting down of sediments 

 derived from the lands, but much of the area disturbed is outside of 

 the zone where continental sedimentation is occurring. On land 

 we commonly consider vulcanism as the accompaniment of diastro- 

 phic movements. It is not altogether unreasonable to suppose that 

 the same is true of the ocean basins. 



It has been said in regard to submarine diastrophic movements 

 that once these ranges were formed there would be nothing to put 

 them out of existence since there is no erosion on the ocean bottom.^ 

 These ranges might then be thought to attain great heights and to 

 produce great irregularities. Isostatic adjustment may have pre- 

 vented the development of these ranges to very great elevations. 

 They might go out of existence or be very much reduced in elevation 

 either by the shifting of the zone of shortening with a resulting col- 

 lapse, by the transference or cooKng of a magma body beneath them, 

 or in the other ways suggested in a previous article.^ 



Several things must be borne in mind in regard to sub-oceanic 

 diastrophism. It would not be as concentrated as on the lands 

 because there could be no geosynclines on the sea bottom, the irregu- 

 larities produced by erosion would be missing. Diastrophism under 



' T. C. Chamberlin, Jour, of GeoL, Vol. XXII, p. 315. 



^ F. P. Shepard, "Isostasy as a Resiilt of Earth Shrinkage," Jour, of GeoL, Vol. 

 XXXI, p. 212. 



