Now the remarkable feature in all these gigantic remains of antiquity is that they are built of brick. In the wide expanse of Mesopotamia plain around there is not a stone quarry to be found. Of Nineveh, we learn from the masterly records of Xenophon that as he was leading the surviving 10,000 Greeks in their retreat from the disastrous field of Babylon back to the sunny Hellespont, some 200 years after the destruction of Nineveh, he came upon a vast desert city on the Tigris. The wall of it was 25 feet wide, 100 feet high, with a 20-foot basement of stone. This was all that was left of Kalah, one of the Assyrian capitals. A day's march farther north he came on another deserted city with similar walls. These were the dry bones of Nineveh, already forgotten and forsaken. Two centuries had in these early ages been sufficient to blot out the memory of Assyrian greatness so completely that Xenophon knew not of it, nor recognized the place where his foot was treading. Barely seventy years ago was the memory of them restored to man, and tokens of the richness and magnificence of the art which embellished them first given to the world. The mounds representing Nineveh and Babylon are some of them of enormous size. The mound of Mugheir (the ancient Ur) is the ancient platform of an Assyrian palace, which is faced with a wall 10 feet thick of red kiln-dried bricks cemented with bitumen. Some of these platforms were raised from 50 to 60 feet above the plain and protected by massive stone masonry carried to a height exceeding that of the platform. But the Babylonian mound of Birs Nimrud, which rises from the plain level to the blue glazed masonry of the upper tier of the Ziggurat, is altogether a brick construction. The debris of the many-coloured bricks now forms a smooth slope for many feet from its base; but above, where the square blocks of brickwork still hold together in scattered disarray, you may still dig out a foot-square brick with the title and designations of Nebuchadnezzar imprinted on its face. These artificial mounds could only have been built at an enormous cost of labour. The great mound of Koyunjik (the palace of Nineveh) covers an area of 100 acres and reaches up 95 feet at its highest point. It has been calculated that to heap up such a pile would "require the united efforts of 10,000 men for twelve years, or 20,000 men for six years" (Rawlinson, _Five Monarchies_), and then only the base of the palace is reached; and there are many such mounds, for "it seems to have been a point of honour with the Assyrian Kings that each should build a new palace for himself" (Ragozin, _Chaldaea_).