that time minor basins have been produced within the greater basins. Throughout the Tertiary era and perhaps even during Quaternary tunes the basins have been the receptacle of the waste from the mountains, which has now deeply 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 rising 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 passes, 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 Alps 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 differences 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 form basins, erosion kept pace with the uplift, and the streams which crossed the rims of the basins preserved uninterrupted channels from the interior to the exterior. 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 any 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 dry, on the contrary, 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 preserved. 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 preservation is dependent on a dry climate. Such a climate, it is true, is often due in large