covered northern India and part of Thibet. In that sea were deposited the thick beds of limestone which are now found in some of the western mountain ridges.
Again in the Miocene period, the forces of distortion within the earth accumulated to such strength that they were able to repeat the mashing and folding, but this time the area affected lay farther to the west and south. At the same time, or perhaps earlier, the eastern part of China was cracked in various directions; and the intervening blocks, settling somewhat unevenly upon their bases, left a group of escarpments and depressions comparable to those now to be found in western Nevada and southern Oregon. As before, the work of erosion and the leveling of the surface was at once accelerated, so that even before the deformation had spent itself the blocks were deeply scarred. It is uncertain how far this period of erosion succeeded in reducing China to base-level. The consummation may have been prevented by gentle warpings of the surface, rising very slowly here and sinking there. When compared with the great breadth of the areas affected, these changes of level seem very slight, but they are nevertheless sufficient to cause great changes in the aspect of the country.
It is one of the basal principles of physiography that streams tend to produce in their channels an almost uniform slope from their headwaters to the sea. If any part of the channel is so flat that the stream is too sluggish to carry sediment, it is built up until it reaches the re-
