The topography of this vast region is naturally divided into two distinct areas by the Kiayü kwan at the end of the Great Wall; one a fertile, well-watered, populous country, differing _toto cœlo_ from the sandy or mountainous wildernesses of the other. The eastern portion is further partitioned into two sections by the ranges of mountains which cross it nearly from south to north in parallel lines, dividing the basins of the Wei and Yellow Rivers near the latter. The passage between them is over the Făn-shui ling, not far from the Tao ho and by the town of Tihtao, leading thence up to Lanchau. This part of the province, watered by the Wei, resembles Shansí in fertility and productions, and its nearness to the elevated ranges of the Bayan-kara induces comparatively abundant rainfall. The streams in the extreme south flow into Sz’chuen, but furnish few facilities for navigation. The affluents of the Yellow River are on the whole less useful for irrigation and navigation, and the four or five which join it near Lanchau vary too much in their supply of water to be depended on. The peculiar feature of Kansuh is the narrow strip projecting like a wedge into the Tibetan plateau, reaching from Lanchau northwesterly between the Ala shan and Kílien shan to the end of the Great Wall. This strip of territory commands the passage between the basin of the Tarim River and Central Asia and China Proper; its passage nearly controls trade and power throughout the northern provinces. The Ta-tung River flows on the south of the Kílien Mountains, but the travel goes near the Wall, where food and fuel are abundant, a long distance beyond its end--even to the desert. The roads from Sí-ngan to Lanchau pass up the King River to Pingliang and across several ranges, or else go farther up the River Wei to Tsin chau; the distances are between 500 and 600 miles. From Lanchau one road goes along the Yellow River down to Ninghia, a town inhabited chiefly by Mongols. Another leads 90 miles west to Síning, whither the tribes around Koko-nor repair for trade. The most important continues to Suhchau, this being an easier journey, while its trade furnishes employment to denizens of the region, whose crops are taken by travellers on passage; this road is about 500 miles in length. Its great importance from early days is indicated by the erection of the Great Wall, in order to prevent inroads along its sides, and by the fortress of Kiayü, which shuts the door upon enemies.