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 Ica. 
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 
