824 



SCIENCE. 



[N. S. Vol. II. No. 50. 



Among the various members of tlie Weesen- 

 steiner grauwacke (Devonian) formation is an 

 interesting series of gneissoid rocks between 

 Goppeln and Tronitz. TKey are feldspathic 

 and full of cordierite, crystals of the latter 

 mineral often attaining great size. 



The diabase sheets which generally lie inter- 

 bedded among the slates and conglomerates are 

 amphibolized, and the diabase tuffs are altered 

 to Actinolite schists. 



Further may be mentioned the metamorphic 

 limestones and their associated ore bodies. The 

 chief interest of the region lies in the diversity 

 of the rocks aifected by contact metamorphism. 



GEOLOGICAL CONFERENCE OF HARVARD UNI- 

 VERSITY, NOVEMBER 19, 1895. 



Theories of Ocean Currents. By W. M. Davis. 



The suf&ciency of difference of equatorial and 

 polar temperatures to cause a convectional cir- 

 culation in the ocean has been strongly disputed 

 by many under Croll's leadership, but warmly 

 upheld by others, notably by Carpenter and 

 Ferrel. The following arguments bear on the 

 discussion : 



The cross-equator current of the Atlantic, 

 flowing obliquely from the South Atlantic eddy 

 to the North Atlantic eddy, continually carries 

 a great volume of water from one hemisphere 

 to the other. The only available path for its 

 return is as an undercurrent. Assuming that 

 the surface currents are wind-driven, and that 

 there is no other cause for movement of deep 

 waters than wind-driven surface currents, it fol- 

 lows that the movement of the deep Atlantic 

 water should cross the equator from north to 

 south. But the distribution of bottom tem- 

 peratures shows very clearly that the bottom 

 movement here is from south to north. Hence 

 .the assumption that no cause but surface wind 

 is operative cannot be permitted, and the most 

 available' other cause is gravitative convection. 



On the other hand, the annual variation of 

 velocity in the surface currents favors their 

 direct control by surface winds rather than 

 their indirect control by convection, as argued 

 by Ferrel. For, if moving as part of a convec- 

 tional circulation, they should move fastest 

 when the poleward temperature gradient in the 

 ocean water is strongest; and this is in late 



summer, when the heat equator of the ocean 

 lies poleward of the geographic equator, and the 

 total difference of equatorial and polar tem- 

 peratures is found in the minimum distance. 

 But if driven by the winds the surface currents 

 should move fastest in winter, for then the 

 poleward temperature gradient in the atmos- 

 phere is strongest and then the winds blow 

 fastest. As far as facts are reported, the east- 

 ward surface drift of ocean waters in the tem- 

 perate zones is strongest in the winter season. 

 The critical point in this argument turns on the 

 essential constancy of temperature in the polar 

 oceans, on account of which the variation of the 

 poleward temperature gradient in the water 

 depends only in the position of the oceanic heat 

 equator; while in the atmosphere the polar tem- 

 perature changes greatly with the season, and 

 hence, in spite of the greater distance from 

 heat equator to pole in the winter hemisphere, 

 the gradient is then strongest on account of the 

 great winter increase in the polar and equatorial 

 temperature contrast. Oceanic convection 

 should be strongest in the summer hemisphere, 

 but atmospheric convection and wind-driven 

 currents in the winter hemisphere. (Fuller 

 publication in the Proceedings, Boston Society 

 of Nat. Hist.) 



T. A. Jaggar, Jr., 

 Recording Secretary. 



NEW BOOKS. 



The Structure and Life of Birds. F. W. Headlet 

 London, Macmillan & Co. 1895. Pp. xx-|- 

 412. 



Milk, Its Nature and Composition. C. M. AcK- 

 MAN. London, Adam and Charles Black. 

 New York, Macmillan & Co. 1895. Pp. 

 xiv-(-180. $1.25. 



Geological Biology. Henry Shaler Williams. 

 New York, Henry Holt & Co. 1895. Pp. 

 xix+395. 



Cambridge Natural History, Vol. V. Peripatus, 

 Adam Sedgwick; Myriapods, F. G. Sinclair ; 

 Insects, David Sharp. London and New 

 York, Macmillan & Co. 1895. Pp. xi+584.' 

 $4.00. 



