GEOLOGY OF PERU—ADAMS. 387 
the Pacific slope of the Andes. This is largely determined by eleva- 
tion and temperature, and is indicated as one travels from town to 
town by the character of the roofs of the houses of the natives. The 
writer in drawing the line upon his published maps®@ used this as a 
basis for his observations and inquiries in order to obtain reliable 
information. 
Near the Gulf of Guayaquil, where the zone of rainfall is deflected 
to the westward from the slope of the Andes over the coastal plains 
and to the Pacific Ocean, the division between coast and sierra would 
be made by continuing the trend of the line into Ecuador, taking 
into consideration the character of the agriculture, which varies 
with the temperature dependent on elevation. For Peru the distinc- 
tion based on climate holds fairly well, but in Ecuador it is less satis- 
factory, since under the Equator and in a region of rainfall the zones 
of vegetation and agriculture do not correspond with the topographic 
distinction also implied. 
Divisions of the coastal region. 
The coastal region of Peru may be divided into plains areas and 
mountainous areas. The plains, according to their geographic posi- 
tions in the country, may be called the “ northern,” “ south central,” 
and “ southern.” Between the northern and south central plains, and 
likewise between the south central and southern, the coast is moun- 
tainous. The northern and south central plains extend inland from 
the shore of the Pacific, but the southern plains are separated from 
the sea by a coast range of hills. The mountainous divisions of the 
coast are diversified by the stream valleys and their tributary dry 
valleys and present a very broken topography. The southern one of 
these two mountainous areas, considered as a mass, rises abruptly 
from the sea and presents many aspects of a dissected plateau. The 
northern area is characterized by a more broken coast line and the 
mountains rise in a ragged, irregular way toward the sierra. It 
would seem to be an open question as to whether these mountainous 
areas should be classed with the coast or the Andes region. Along 
the inner border of the plains are the “ foothills ” rising to the sierra, 
and at a corresponding distance inland in the mountainous divisions 
of the coast there is a transition zone known as the “ valley heads of 
the coast” (cabezeras de los valles), where the valley floors become 
narrow and stony, so that the agriculture of the coast is impossible, 
and the mountains rise on either side into the temperature and cli- 
mate of'the sierra. 
@ Maps reproduced in this report as Plates Nos. 2, 3, 4, and 5. 
