246 



SCIENCE 



[N. S. Vol. XXXI. No. 790 



the superficial layers in freezing. Were 

 the more profound causes of the climatic 

 state so modified as to ameliorate the sever- 

 ity of polar cold, both of these influences 

 would be moderated, and the effects of 

 freshening by rivers and precipitation 

 would not be offset to the extent that they 

 now are. 



On the other hand, equatorial waters are 

 warmed and evaporated, and they are thus 

 rendered light because warm, yet heavy 

 because saline. The actual density, as 

 compared with that of polar waters, is now 

 less than the latter, but both observation 

 in the oceans and calculation show that the 

 balance is small. Were the polar waters 

 less chilled or more freshened or both, the 

 equatorial waters would be heavier, and 

 the reversed circulation suggested by 

 Chamberlin must result. 



The cold of the present polar climates is 

 extreme and unusiial. To whatever funda- 

 mental causes we may attribute it, we know 

 that it did not exist during the Miocene, 

 Eocene, Cretaceous, later Jurassic, Carbon- 

 iferous, Devonian, Silurian, Ordovician, or 

 later Cambrian. Frigid conditions may 

 have occurred with severity in the earlier 

 Jurassic or Triassic and in the early Cam- 

 brian or late pre-Cambrian. That is to 

 say at periods which, like the present, were 

 periods of exceptional continental expan- 

 sion and elevation. It seems to follow 

 cogently that the condition of oceanic cir- 

 culation which depends upon polar cold is 

 also exceptional. Under more genial condi- 

 tions, the waters in high latitudes would be 

 lighter than now because warmer. They 

 would also be more generally freshened by 

 precipitation, and nowhere rendered more 

 saline by freezing. The conditions which 

 now occasion the greater density of polar 

 waters would thus fail and the balance 

 would sink on the side of the equatorial 

 waters. Heavier equatorial, lighter polar 



waters have probably been the normal con- 

 dition; the reverse, which now exists, the 

 abnormal. 



This conclusion follows entirely apart 

 from the consideration that the extraordi- 

 narily mild climates of some ages are 

 rendered less difficult to understand if the 

 deep-seated circulation of the ocean were 

 thus reversed, if it had normally been a 

 movement of warm waters in the depths, 

 instead of at the surface, toward the pole. 

 But the facts, which it explains, strengthen 

 the hypothesis and place it in the front 

 rank of important suggestions in the study 

 of paleogeography. 



PEEIODICITY OP DIASTROPHISM 



Diastrophism, the process which com- 

 prises all movements of the earth's crust 

 that modify continents or give rise to 

 mountain ranges, has been characterized 

 by periods of activity in alternation with 

 periods of quiescence, throughout all geo- 

 logic history. 



This principle of the periodicity of earth 

 movements rests upon the observation that 

 periods when continents emerged from the 

 sea and became mountainous have alter- 

 nated with periods when continents had 

 become low and were extensively sub- 

 merged. 



The emergence of continents and the 

 growth of mountains are due to activity of 

 the internal terrestrial forces; the reduc- 

 tion by erosion to low lands and the resub- 

 mergence mark the period of inactivity. 

 The Quaternary is a time of decided ac- 

 tivity, resulting in large continents and 

 great mountain chains. It has been pre- 

 ceded by times of relative quiescence and 

 by others of greater movement, in alterna- 

 tion, as far back in the past as the record 

 goes. 



While geologists in general will agree 

 that this is a true principle, they find it 



