The second building is at Karkh, the town known in Syriac as Karkhâ de Lâdan. It was founded by Shapûr II (309--379)[181] when he rebuilt Susa, from which it is not far removed. Of this palace we have nothing but a fragment, possibly a monumental entrance (Fig. 20). The central chamber is covered by a dome which was set over squinches upon four wide archways.[182] The cutting away of the walls under a dome is thus very highly developed at Karkh. Four transverse arches span each of the wings, and the space between the arches is covered by a vault. In connexion with Ukhaiḍir this scheme of the wings at Karkh is of special interest because it is repeated in room 32, where even the windows under the vaults are reproduced by blind niches. The material used at Karkh is brick, and it may here be noticed that at Susa and in Babylonia, where brick was the only available local material, it is invariably used by Sasanian architects; in Fars and in the Qaṣr-i-Shîrîn district, where stone was more easy to obtain than brick, they constructed in unsquared stones, roughly coursed, using brick only for the larger vaults and domes and for those portions of the walls which were finely finished. The latter system was employed at Ukhaiḍir. Vault construction in stone was facilitated there by the fact that the stone broke naturally into thin slabs and could be made to assume more or less the proportions of brick tiles. For this reason stone vaults could be built without the use of centering. At Qaṣr-i-Shîrîn this was not the case. The stones are smooth rounded blocks like large pebbles; it would have been impossible to use them for vaults unless the cement in which they were laid had been peculiarly strong, and the vaults thus formed are of the rudest kind. Coursed and undressed stone held together by a clay mortar was used for vaulting purposes as early as Lydian times; a vault of that character covers the tomb chamber of the tumulus of Alyattes near Sardis. The same masonry is found in the terrace of the Takht-i-Mâder-i-Suleimân at Pasargadae (fifth century B.C.), and is still in common use in Asia Minor.[183] Masonry of dressed and undressed stones set in a mortar of clay or pitch has been found in Assyrian buildings,[184] but gypsum mortar was not known in Mesopotamia till the seventh century. Its earliest appearance was in the palace of Nebuchadnezzar at Babylon. In Egypt it is of much earlier occurrence, and the use of mortar in the Aegean region during the second millennium B.C. (Mycenae, Argos) was probably due to Egyptian influence.[185] Hatra is the earliest Mesopotamian monumental building in dressed stone and mortar; it was an example which was not followed by Sasanian architects. The method was foreign to local tradition; native workmen returned to their own systems and continued to construct wall, vault, and dome of brick or of undressed stone.