42 



PRINCIPLES OF STRATIGRAPHY 



Thus a poleward niovcnient of the upper air and an equatorial 

 movement of the lower air are estahlished, with a rising of the air at 

 the equator and a descent in the polar regions. (Fig. 5.) This is the 

 fundamental type of circulation of the atmosphere. This simple type 

 of circulation is greatly modified, however, at any rate so far as the 

 lower part of the atmosphere is concerned, by various factors, chief 

 among which may be counted the rotation of the earth, the distribu- 

 tion of land and sea, the lateral pressure exerted by the expanding 

 warm air of the equatorial region, and the crowding from the 

 descent of the air in high latitudes, the subsidiary motions set going 



Fig. 5. Diagrammatic section 

 of the earth and the en- 

 closing atmosphere, illus- 

 trating the fundamental 

 type of circulation in the 

 lower atmosphere, estah- 

 lished by unequal heating, 

 as a result of differences 

 in latitude. (After Salis- 

 bury-Physiography. ) 



Fig. 6. Diagram showing the general 

 movements which would take place 

 in the lower air if there were no 

 rotation. (After Salisbury.) 



by the aspiratory action of the larger currents, etc. Considering 

 that in each hemisphere there is, on the equatorial side, a rising 

 mass of air, which turns poleward, while on the poleward side an 

 equivalent current descends, and then flows equatorward, we have 

 an intermediate area around which this circulation takes place, and 

 these areas are belts of high pressure. Their location is approxi- 

 mately in latitudes 30° N. and S., a location determined in part 

 by the rotation of the earth. The center of this high pressure belt 

 in the northern hemisphere is in 30° N. lat., or a little less in Jan- 

 uary, with great expansion of the belt northward on the land. In 

 July this center shifts north to about lat. 35° and the belt is inter- 

 rupted on land. The reverse is true for the southern hemisphere, 

 where the summer conditions exist in January, with the center of 



