GEOLOGY OF PERU— ADAMS. 



389 



are the great inter-Andean valleys, which are occupied by streams 

 tributary to the Amazon and which are shown in a general way 

 on the hachured map, and which may be named from the rivers 

 occupying them. On the Pacific slope there is one inter- Andean val- 

 ley between the Cordillera Negra and Blanca known as the " Valley 

 of Huaylas " (Callejon de Huaylas). In addition should be noted 

 the Titicaca Lake basin. If one attempts to go further into the 

 classification of the physiographic features, there are many short 

 ranges of mountains or spurs from the main Cordillera, some of 

 which are named on Raimondi's map, and also high plains and table- 

 lands (frequently called "punas") which are worthy of distinction. 



Fig. 2. — Variations of temperature at lea. 



Rainfall in the Andes. 



The rain which falls in the Andes region is brought as vapor from 

 the Atlantic and most of it is precipitated in the Amazon region or 

 on the eastern flank of the first Cordillera which it encounters. Dur- 

 ing the summer season the clouds rise higher and pass farther to the 

 west, distributing their moisture on the Cordilleras and a part of it 

 crosses the Continental Divide or the western Cordillera. It is gen- 

 erally believed that the rainfall on the Pacific slope, the limit of 

 which has already been discussed, comes over the Cordilleras, except 

 in the region of the Gulf of Guayaquil. This is in accordance with 

 the observations of many travelers and the general theory of the 



