The cutting on the cylinders, or rather on all the engraved stones of western Asia, is in intaglio. This is the earliest form of engraving upon _pietra-dura_ in every country; the cameo is always a much later production; it is only to be found in the last stage of development, when tools and processes have been carried to perfection. It is much easier to scratch the stone and then to add with the point some definition to the figure thus obtained, than to cut away the greater part of the surface and leave the design in relief. The latter process would have been especially difficult when the inscriptions borne by many of the seals came to be dealt with. What long and painful labour it would have required to thus detach the slender lines of the cuneiform characters from the ground! And why should any attempt of the kind be made? As soon as these engraved stones began to be used as seals, there was every reason why the ancient process should be retained. The designs and characters impressed upon deeds and other writings were clearer and more legible in relief than in intaglio. And it must be remembered that with the exception of some late bricks on which letters are raised by wooden stamps, the wedges were always hollowed out. We find but one period in the history of Chaldæa when, as under the early dynasties of Egypt, her written characters were chiselled in relief. It is, then, apparent that the artists of Chaldæa would have done violence to their own convictions and departed from long established habits, had they deserted intaglio for work in relief. That they did not do so, even when their skill was at its highest point, need cause us no surprise. The Chaldæans naturally began with the softest materials, such as wood, bone, and the shells picked up on the shores of the Persian Gulf. Fragments of some large pearl oysters and of the _Tridacna squamosa_, on which flowers, leaves, and horses have been engraved with the point, have been brought from lower Chaldæa to London (see Fig. 138).[301] Limestone, black, white, and veined marble, and the steatite of which most of the cylinders are made, were not much more difficult. These substances may easily be cut with a sharp flint, or with metal tools either pointed or chisel-shaped. With a little more effort and patience still harder materials, such as porphyry and basalt; or the ferruginous marbles--serpentine, syenite, hematite--could be overcome. The oldest cylinders of all, those that are attributed to the first Chaldæan monarchy, are mostly of these stubborn materials; their execution was easy enough to the men who produced the statues of Gudea.[302] All that such men required to pass from the carving of life-size figures to the cutting of gems was good eyesight and smaller tools.