OUTLINE OF THE REPORT. 223 
that time minor l)asiiis have been produced within the greater basins. Throughout 
the Tertiary era and perhaps even during Quaternary times the basins ha\-e been the 
receptacle of the waste from the mountains, which has now deeph- filled their lower 
portions. The waste thus deposited lies horizontal in the centers of the basins, but 
is warped along the edges, the older strata being more warped than the younger. 
Apparently the basins have been subjected to a gradual process of intensification by 
which they have become deeper, while their edges have been folded and uplifted. 
The large basin of Iran, with its rim of lofty mountains, is a typical example 
of long-continued erosion and deposition in a mountainous plateau under arid 
conditions. The traveler is wearied by an endless repetition of naked mountains 
vising on the edges, or even in the midst of smoothly-floored basins, in which the 
streams have for ages deposited waste from the mountains. In the centers of many 
basins stretch vast playas, whose smooth expanse is often covered with salt. Where 
water is more scarce fields of drifting sand move slowly forward, while between 
the barren mountains and the plains gently sloping fans of gravel merge into level 
sand and silt on the one hand, while on the other they mantle the flanks of the 
ridges, and even overtop the pa.sses, uniting one basin to another. What few 
streams there are flow toward the basin centers in terraced valleys, and often the 
shores of the playas and lakes are also terraced. 
No one, however unobservant, can fail to be struck by the contrast between 
the physical features of Persia and those of the well-watered countries of Europe 
and America. The only competent cause for this difference seems to be that the 
climate in the two regions is different. It is often assumed that the presence of 
inclosed basins, such as those of Persia, is due to some special variety of warping 
of the earth's crust. That the basins are due to warping can hardly be doubted, 
but there seems to be no reason for thinking that the warping is of a peculiar sort. 
The mountain building of the Al])s and the Himalayas must have given rise to 
basins just as did the mountain-building of Persia and of the basin region of 
North America. The present diflerences are due to differences in climate. In the 
Alps one of two things happened : First, the streams may have eroded so fast that 
when tectonic forces began to uplift the mountains and fonn basins, erosion kept 
pace with the uplift, and the streams which crossed the rims of the basins pre- 
served uninterrupted channels from the interior to the e.xterior. Second, a newly 
formed basin may have been filled by a lake, the overflow of which soon cut a 
channel so deep as to drain off all the water, or at least all that stood at anv great 
elevation above sea-level. In either case the basins were quickly converted into 
valleys opening freely toward the sea. In Persia, where the climate is dr>-, on 
the contrar}', the erosion of the scanty streams was insufficient to keep pace with 
the movements of warping, and inclosed basins were produced in which the streams 
still terminate in salt lakes, playas, or gravel fans, where all the material that 
comes from the mountains is preser\-ed. A peculiar topography was produced, 
which consists of inclosed basins within which are huge gravel slopes, broad plains 
of silt, buried mountains, .salt lakes, and fields of sand-dunes. In brief, basins are 
the universal accompaniment of mountain-building, but their preser\-ation is 
dependent on a dry climate. Such a climate, it is tnie, is often due in large 
