LII REPORT OF THE BUREAU OF ETHNOLOGY. 



lyzecl liy the diminishing decHvit}' of their ways, and some of 

 their valleys Avere converted into basins, while others were 

 robbed by the transgression of the more active streams flow- 

 ing- southwestward. In the southwestern half of the tract the 

 rainfall remained slight, and the feeble streams bom of the 

 rare storms spent their energy in carrying debris from the 

 mountains into the valleys, whereby the area of desert plains 

 was still further increased. To this series of movements 

 many of the peculiarities of the region are due; excepting 

 the Little Colorado (which has been affected by peculiar 

 conditions) and its tributaries, the principal streams flow west- 

 ward, southward, and southwestward; their waters gather in 

 the mountains or northeastern plateaus, and they flow for a 

 time through canyons which gradually diminish in depth as 

 the streams approach tide level — for the mean slope of the 

 surface is greater than the mean slope of the stream; and dur- 

 ing the dry season and sometimes througliout the year the 

 streams are smaller in the lower courses than in the upper 

 regions — for the waters are drank by the thirsty soil 

 and absorbed by the heated air. South of the Gila and all the 

 way to Rio Yaqui, halfway down the Gulf of California, the 

 j)arclied land yields no water to the sea. In their upper 

 reaches the streams corrade, in their lower courses they deposit 

 the debris gathered toward their sources; they degrade above 

 and aggrade below, and thereby the great geologic process of 

 gradation is in this region completed without the aid of the sea, 

 save as a source of vapor. So the southwestern part of the 

 tract is a region of aiid plains of aggradation, beneath which 

 the Mesozoic and Cenozoic formations are largely buried; the 

 northeastern part is a region of arid plateaus, in which these 

 formations crop out over the surface and in rugged canyon 

 walls; while the central portion is a broad zone, in which the 

 later formations crop out in low jilateaus and mesas, and in 

 which the southwestward-flowing streams are often flanked by 

 alluvial terraces and floodplains. These geographic condi- 

 tions, originating in clearly defined geologic processes, have 

 aff"ected the habitability of the tract since men first appeared 

 therein — indeed, to these conditions the peculiarities of south- 

 western aboi'iginal culture are to be ascribed in large measure. 



