means of difference of temperature. The effect must be to pro- 

 duce a constant ascent of the equatorial column and an inflow of 

 cold water below equal to the outflow above. In short, the wind 

 must produce a system of circulation precisely the same as that 



of the ^ 



ference of water from the top of the one column to the top of t 

 other. This vertical motion is tlierefore as much a necessary c( 

 sequence of the wind theory as it is of the gravitation theory. 



II. Geology awd Mineralogy. 



1. On the Gravel and Cohhle-s 

 the Middle States; by Wm. B. Rogees. (I 

 Hist., vol. xviii, 1875, 101. — The deposits here described as occur- 

 ring in the great river valleys and on the adjoining slopes, at 

 Richmond, Ya., Washington, D. C, and elsewhere, consist chiefly 

 of layers of water-worn gravel and stones, with ferruginous sands 

 and clays. In most localities the larger pebbles are foxmd in the 

 upper part of the deposit; but in others, as at Alexandria and 

 Richmond, the cobble-stone deposit is overlaid by bedded sands 

 and gravel. Casts of Scolithus occur in some of the pebbles col- 

 lected at Washington and Richmond. The deposit at Washington 

 covers the entire plain on which the city is built ; it averages 75 

 feet in height above mean tide, but rises on the north to about 100 

 feet. Thence it spreads over the slopes, covering the grounds of 

 Columbian College, and the higher hill of the Soldiers' Home, over 

 200 feet above sea level. In the vicinity of the Capitol the stones 

 are often a foot in diameter, and near Georgetown in a recent ex- 

 cavation some are much larger. The facts point to transporta- 

 tion along the valleys, but by streams of much greater width than 

 those now there. The distaiace transported may be learned from 

 the fact that the nearest Potsdam or Scolithus sandstone to Rich- 

 mond is 80 miles, and along James River 160 miles ; and that from 

 Washington to the western side of the Blue Ridge is 40 miles, and 

 along the Potomac 50 to 60 miles. Prof Rogers remarks on the 

 origin of these deposits as follows : 



" Speculating on the causes by which these deposits have been 

 formed, it may, on the one hand, be imagined that during the Glacial 

 period the icy covering of the north and west prolonged itself in the 

 valleys of the great rivers, as far south as the James, and even the 

 Roanoke River, bringing down to the belt of land now marking the 

 ■ (»f tide water debris from the Appalachian rocks, mingled v '"^ 



marking, and gave to the whole deposit the distribui 

 fication which it now presents; or, on the other hand, it may \ 

 ceived that the transporting force of the rivers themselves, s^ 

 and rapid as they must have been in the closing ages of the C 



