The general principle on which the sea or main walls are constructed is usually the same, though the various ahu differ greatly in appearance: first comes a row of foundation blocks on which have been set upright the largest stones that could be found; the upper part of the wall is composed of smaller stones, and it is finished with a coping. The variety in effect is due to the difference in material used. In some cases, as at Tongariki (fig. 33), the most convenient stone available has consisted of basalt which has cooled in fairly regular cubes, and the rows are there comparatively uniform in size; in other instances, as at Ahu Tepeu on the west coast (fig. 37), the handiest material has been sheets of lava, which have hardened as strata, and when these have been used the first tier of the wall is composed of huge slabs up to 9 feet in height. Irregularities in the shape and size of the big stones are rectified by fitting in small pieces and surmounting the shorter slabs with additional stones until the whole is brought to a uniform level; on the top of this now even tier horizontal blocks are laid, till the whole is the desired height (fig. 42). The amount of finish put into the work varies greatly: in many ahu the walls are all constructed of rough material; in others, while the slabs are untouched, the stones which bring them to the level and the cubes on the top are well wrought; in a very few instances, of which Vinapu (fig. 35) is the best example, the whole is composed of beautifully finished work. Occasionally, as at Oroi, natural outcrops of rock have been adapted to carry statues (fig. 122). [Illustration: FIG. 37. AHU TEPEU. Part of seaward wall showing large slabs—some of the stones forming upper courses are wrought foundation-stones of canoe-shaped houses, pp. 215–16. ] [Illustration: FIG. 38. METHOD OF EXPOSING THE DEAD FROM ILLUSTRATED DESCRIPTIONS. ]