46 



PRINCIPLES OF STRATIGRAPHY 



A glance at a chart of isobaric lines for January shows that 

 Eurasia is a center of high pressure, and hence of air depression. 

 The Indian Ocean is an area of low pressure and hence strong con- 

 tinental winds flow southward, and by the rotation of tiie earth are 

 deflected to the west. Hence, over India we have northeast conti- 

 nental winds which harmonize with the trade winds and augment 

 their effectiveness. In summer, on the other hand, Eurasia is a 

 center of low pressure, centering in July over northwestern India, 

 and hence the direction of the winds is a northward one, deflected 

 to the northeast by the rotation of the earth. This is diametrically 

 opposed to the direction of the northeast trade winds, which become 



Fig. II. Isobars of India for August. 

 (After Bartholomew; from Sal- 

 isbury.) 



Fig. 12. Winds of India in midsum- 

 mer. (After Koppen; from Sal- 

 isbury.) 



displaced by this more powerful southwest monsoon. The influ- 

 ence of these changes on the ocean currents will be noted later. 

 Spain, which lies in the zone of westerly winds, furnishes another 

 illustration of the importance of seasonal winds or monsoons. In 

 winter the central plateau is characterized by low temperature and 

 high pressure, and the winds blow outward, while in summer the 

 reverse is true, the winds blowing toward the center. The impor- 

 tance of such winds in affecting the relative humidity of the climate 

 and corresponding sedimentary deposits must not be overlooked. 



Cyclones and Anticyclones. 



An area of high barometric pressure produces an anticyclonic 

 movement of the air, which passes out in all directions from it. In 

 the northern hemisphere these winds are deflected to the right, 

 making a clockwise circulation, while in the southern hemisphere 

 deflection to the left produces a counter-clockwise circulation. Con- 

 versely, the low pressure areas are characterized by a cyclonic cir- 



