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SCIENCE 



[N. S. Vol. XXVII. No. ( 



as details of distribution of land and sea at 

 any particular epocli are concerned; but there 

 is no doubt as to the general character of the 

 changes which have occurred from period to 

 period, and these changes are of very notable 

 degree. It was shown that we could distin- 

 guish at least four great diastrophic cycles, 

 each one of which consisted of an initial epoch 

 of pronounced continental emergence associ- 

 ated with mountain building, and a later 

 period of planation of lands to a peneplain 

 condition and very wide marine transgression. 

 Of these four cycles, the latest is that in which 

 we live, and we are at the present time in the 

 initial epoch of that cycle which is marked 

 by decided mountain features. If it be as- 

 sumed that the volume of the oceanic waters 

 has not varied materially throughout the geo- 

 logic ages which these cycles comprise (and 

 the physical evidence substantiates this as- 

 sumption) it follows that changes in the 

 superficial extent of seas and lands must be 

 the result of changes in the shape and depth 

 of oceanic basins ; but such variations of form 

 can only proceed from movements of the litho- 

 sphere, movements which have the character 

 of warping; and as such variations are traced 

 throughout the entire legible period of geol- 

 ogic history, it follows that movement has 

 been an equally persistent factor. From the 

 nature of the effects, which as stated in the 

 early part of the cycle are more vigorous and 

 in the later part more gradual, it follows that 

 there is a pronounced variation and a certain 

 periodicity in the movements; but we are not 

 able to distinguish any time when epeirogenic 

 change has not been in progress in some per- 

 ceptible degree. 



Tailing up the other aspect of diastrophism, 

 namely, erogenic or mountain-building move- 

 ments, the speaker showed a map of the world 

 on which were delineated the axial trends of 

 great folded mountain chains. Prom a study 

 of their distribution and of their relations to 

 areas of maximum elevation and denudation 

 it appears (a) that the Atlantic and Pacific 

 oceanic basins have widened at the expense of 

 the continental margins which have been 

 crushed in; and (b) that the whole southern 



hemisphere (the oceanic hemisphere) has 

 spread northward at the expense of the north- 

 ern (continental) hemisphere. The very ex- 

 tensive areas about the Arctic Ocean, which 

 have remained undisturbed since an early 

 Paleozoic time, have formed a central mass 

 against which the more southern portions of 

 Eurasia and North America have been folded. 

 Prom the detailed consideration of these 

 erogenic movements Mr. Willis concludes that 

 the great tangential displacements are due to 

 an expansion of the suboceanie masses. This 

 conclusion is independent of any particular 

 hypothesis of the causes of such expansion, 

 but in his opinion the efi'ects are so distributed 

 as to be inconsistent with any interpretation 

 on the lines of the contraction theory. Fol- 

 lowing Button, Gilbert and Hayford in the 

 general concept of isostatic adjustment toward 

 an equilibrium among denser and lighter 

 masses of the lithosphere, he holds that the 

 persistent epeirogenic movements as well as 

 the occasional erogenic movements are due to 

 strains set up among the heterogeneous bodies 

 of the earth's crust which differ from one 

 another in density. It appears obvious that 

 were the suboceanie masses enough denser 

 than the subcontinental masses, the former 

 would displace the latter, provided the firm- 

 ness of rocks be insufficient to maintain their 

 form. In masses of large dimensions, cubes 

 measured by tens of miles on the side, rocks 

 are not firm enough, even under the pressures 

 that exist down to a depth of 100 miles below 

 the surface, to maintain their form if unsup- 

 ported at the sides ; and thus there must exist 

 throughout the lithosphere a tangential strain 

 or tendency to spread which is somewhat 

 greater in the denser than in the lighter 

 masses. This strain is apparently not ini- 

 tially sufficient to cause movement if we may 

 assume that the lithosphere were at any par- 

 ticular instant in a state of equilibrium; but 

 it is a persistent strain of constant direction 

 in any particular locality, and in the event of 

 disturbance through changes of molecular ar- 

 rangement or temperature, it would serve to 

 direct the effects of any growing strain and 

 ultimately to cause movement in the effective 



