Babylonia is a treeless country, and timber had to be imported from the earliest times. The date palm was probably introduced by man, as were certainly the vine and the fig tree, which were widely cultivated, especially in the north. Stone, suitable for building, was very scarce, and limestone, alabaster, marble, and basalt had to be taken from northern Mesopotamia, where the mountains also yield copper and lead and iron. Except Eridu, where ancient workers quarried sandstone from its sea-shaped ridge, all the cities were built of brick, an excellent clay being found in abundance. When brick walls were cemented with bitumen they were given great stability. This resinous substance is found in the north and south. It bubbles up through crevices of rocks on river banks and forms small ponds. Two famous springs at modern Hit, on the Euphrates, have been drawn upon from time immemorial. "From one", writes a traveller, "flows hot water black with bitumen, while the other discharges intermittently bitumen, or, after a rainstorm, bitumen and cold water.... Where rocks crop out in the plain above Hit, they are full of seams of bitumen."[30] Present-day Arabs call it "kiyara", and export it for coating boats and roofs; they also use it as an antiseptic, and apply it to cure the skin diseases from which camels suffer. Sumeria had many surplus products, including corn and figs, pottery, fine wool and woven garments, to offer in exchange for what it most required from other countries. It must, therefore, have had a brisk and flourishing foreign trade at an exceedingly remote period. No doubt numerous alien merchants were attracted to its cities, and it may be that they induced or encouraged Semitic and other raiders to overthrow governments and form military aristocracies, so that they themselves might obtain necessary concessions and achieve a degree of political ascendancy. It does not follow, however, that the peasant class was greatly affected by periodic revolutions of this kind, which brought little more to them than a change of rulers. The needs of the country necessitated the continuance of agricultural methods and the rigid observance of existing land laws; indeed, these constituted the basis of Sumerian prosperity. Conquerors have ever sought reward not merely in spoil, but also the services of the conquered. In northern Babylonia the invaders apparently found it necessary to conciliate and secure the continued allegiance of the tillers of the soil. Law and religion being closely associated, they had to adapt their gods to suit the requirements of existing social and political organizations. A deity of pastoral nomads had to receive attributes which would give him an agricultural significance; one of rural character had to be changed to respond to the various calls of city life. Besides, local gods could not be ignored on account of their popularity. As a result, imported beliefs and religious customs must have been fused and