68 PRINCIPLES OF STRATIGRAPHY 



per year. The cause of this rainfall is the rising and expansion of 

 the air warmed near the surface, and the consequent cooling- and 

 condensation of its moisture. These precipitations are almost daily 

 accompanied by thunderstorms, especially in the afternoon and even- 

 ing. The belt shifts north and south with the position of the sun. 

 Two maxima occur, one in April and one in November, lagging 

 thus somewhat behind the vertical sun, the April maximum being 

 the principal one. The minima between these are seldom rainless. 

 This constitutes the equatorial type of rainfall. The tropical belt 

 of calms, situated in about latitude 30°, but likewise shifting with 

 the position of the sun, is a region of descending and hence com- 

 pressing winds, which thus become warmed and have a drying ef- 

 fect. 



From the shifting nature of the tropical rain belt with the change 

 in position of the sun, the regions near its margin (or more than 

 10° to 12° from the equator) are subject to regular alternations of 

 one wet and one dry season, the two rainy seasons having become 

 merged, owing to the short interval between the periods of vertical 

 sun in summer. This is followed by a single dry winter season. 

 During this period the trade winds produce their drying effect, 

 which continues until the equatorial rain belt again encroaches. This 

 type of alternating abundant rainfall and draught is called the 

 tropical type. The dry (winter) season lasts eight months in 

 typical cases ; the lowlands become parched and vegetation withers 

 away, whereas during the shorter wet season grass and flowers will 

 grow in abundance and all life take on new activity. The Soudan, 

 between the Sahara and the equatorial forest of Africa, partakes 

 alternately of the character of each ; vegetation grows actively when 

 the doldrums are over it (May to August), but when the trade 

 winds blow the ground is dry and parched (December to March). 

 The llanos of Venezuela have a dry season when the trade winds 

 blow, /. €., during the northern winter, and an abundant rainfall 

 from May to October. The reverse is the case in the canipos of 

 Brazil, situated south of the equator, where the rain falls from 

 October to April, while the remainder of the year is dry. The 

 overflow of the Nile results from the abundant rainfall in the Abys- 

 sinian Mountains when the equatorial rain belt migrates northward. 



In the monsoon belts the rains follow the vertical sun, and there 

 is thus a simple annual period similar to that of the tropical type. 

 In summer the warm air flows inland from the sea, bringing much 

 moisture, which is condensed and precipitated as the winds are 

 forced to rise over mountains or highland plateaus. The heaviest 

 precipitation in India is in the Khasi Hills (Assam) at the head of 



