DISTRIBUTION OF THE FISHES 37 
(b) Vuleanism 
Along the borders of the western Andes are large areas in which the details 
of the upthrust formations are blotted out by lava beds and voleanic peaks. In 
Ecuador (outside our area) this also holds good for the Eastern Cordillera, along 
the Atlantic scarp. Central and Northern Peru have few volcanic areas, Southern 
Peru and Northern Chile many. They are largely responsible for the surface con- 
figurations where they occur. 
(ce) Diastrophism 
The latest phases of the upthrust from the paleozoic sea may be seen in the 
Eastern Cordillera of Bolivia, which stands in magnificent relief (22000 feet) above 
Lake Titicaca (12500 feet) with the most youthful contours, and with a snow- 
capped granitic core. Thus the cordilleras are variously crowned with volcanic 
peaks, with sedimentary formations often on edge, or with granitic formations. 
Upon these the winds, frost, rainfall, ete. work their varied effects. 
(d) Aggradation 
On the western slopes we find many minute areas in which water has been the 
principal constructive agent, but few large areas, and no extremely large ones. 
Due to the deep sea close inshore and the small volume of the rivers, delta forma- 
tions are very small. Inland the rivers are of youthful to extremely youthful 
form, with room only for very small playas at infrequent intervals. Such a cove 
in the hollow of the hills may be remote, yet if suited to crops is sure to be oecupied, 
even though of few acres, or even of fractional acreage. The mouths of rivers are 
the largest—the Rimac, the Santa, the Tambo, or the Camani. (See Johnson’s 
aerial photographs. ) 
The eastern slope on the contrary is aggraded terrain, or alluvial land, from 
the Andes to the mouths of the rivers. With a maximum precipitation, and having 
from 2600 to 8000 miles in which to make its final thousand feet of descent, it does 
so with an extremely slight gradient, and the well-known inundation. The valley 
land of the Amazon is the repayment on the old palaeozoic account which the 
Andes are making to Brazil for the materials borrowed from its eastern ranges. 
(Johnson, pages 144-150.) 
The result of the interplay of these varied forces upon the varied substratum 
described above is a profound variation in the geographic and ecological regions 
and subregions. Add to these factors those of climate, elevation, latitude; clothe 
the land with the most diversified vegetation, and we have the background of a 
most heterogeneous fauna, aquatic as well as terrestrial. 
(3) Masor TopoGrapHic FEATURES 
The Andes arise from the depths of the Antarctic, an island at a time, until 
the mainland is reached at the Straits of Magellan. 
