In this seclusion, neither penetrated by bays of the sea nor traversed by mighty rivers, the region exhibits a certain similarity to the highlands of Arabia. The centre of the Iranian land, like that of Arabia, is occupied by a great desert where only nomadic life is possible. But the soil of Iran is more diversified in regard to elevation and depression. The northern half of the land is higher than the southern, the centre is hollowed out in the form of a trough, so that in the east, at any rate, the waters from the inner slopes of the mountainous rim fall into the depression, and collect in fructifying lakes. The oases and fertile valleys are more numerous and extensive than in Arabia, and though the rivers of the inner table-land, like the streams of the northern edge, which flow to the north, are lost in the sand or end in unimportant lakes, they nevertheless render agriculture possible over wide tracts of country. The northern side is more diversified and superior in formation to the south. The southern edge, which sinks down to the ocean, closely resembles Arabia in the climate and the nature of the country; the mountains of the north, on the other hand, exhibit green pastures and splendid forests where Arabia has nothing but bare peaks: in the Hindu Kush, and Elburz on the Caspian Sea, as well as in Aderbeijan, they rise into vast Alpine districts. The eastern edge, extending over a distance of 900 miles, rises like a steep wall out of the valley of the Indus; a few long and difficult passes lead from the Indus to the high ground, which on the north commences with cold bare flats, and on the south with slopes still more desolate and barren, and at the same time intolerably hot. Only the terraces of the valley of the Cabul, which flows down into the Indus, allow a convenient exit towards the north, and present a soil to a great extent so fertile that three harvests can be reaped in the year. The western edge of Iran, on the other hand, is formed by parallel ridges running from the north-west to the south-east, between which, beside extensive mountain pastures, lie narrow and well-watered valleys. In the north-west the low-lying regions are rich in meadows and forest; while those between the abutting ridges of the western and southern edge are warm, and even hot, in climate, rich and luxuriant in vegetation.