EOLIAN GEOLOGY 
173 
erode. On every hand the country clearly shows it. It is equally 
manifest that notable leveling and lowering has gone on at a rapid 
rate. 
Extensive erosion is everywhere manifest, but it is of peculiar 
type. There is little of the sharp incision of the plains-surface 
such as normally characterizes stream-action, especially in a high- 
lying region having slopes of high gradients. Erosion is of the 
broad-basin type — wide, flat-bottomed, even plains between 
abruptly upturned rims against resistant rock-masses. As the 
lava-flows and coulees become more and more numerous the 
separate basins became divided and smaller, but general lowering 
of surface go on without interruption. There can be no ques¬ 
tion but that the lava-capped mesas at varying heights represent 
former levels of the general plains-surface. Such is the difference 
between wind scour and stream corasion. 
Keyes. 
Basinal Sand-Drifting o?i Arid Piedmonts. As streams of 
many desert ranges leave their canyons and open out upon the 
piedmont belt they abruptly undergo some quite remarkable and 
wholly unexpected changes. They appear to be suddenly trans¬ 
formed from channels undergoing rapid degradation to those 
which are notably aggraded. The short, deeply V-shaped profile 
of a canyon becomes widely flat-bottomed. For distances of sev¬ 
eral miles both above and below the line which marks the meeting 
of mountain and intermontane plain the valley floors display uni¬ 
formly graded sand-plains. 
. It is the custom to explain these and associated phenomena upon 
an hypothesis of normal stream-action. In so ascribing them sev¬ 
eral features appear to be either overlooked or largely misinter¬ 
preted. The incongruity of wide sand-plains at the mouth of a 
torrential stream-course that is everywhere characterized by bould¬ 
ers of large size seems never to have presented itself. That these 
sand-plains are the normal result of stream aggradation in the 
final stage of arroyo-running is not a necessary consequence of 
their position, as is so commonly regarded. They seem often, but 
not always, to be associated with the so-called alluvial-fans which 
are premised for each intermittent brook debouching upon the 
desert bolsons. 
The neglected features to which especial attention is here di- 
