Thus some triangular prisms of granite have been found at the foot of the pyramid of Chephren, which seem to have formed part of its lower casing.[221] Such a section seems, upon paper, the simplest that could be adopted for the filling in of the angle between two of the steps, but it is far inferior in solidity to the trapezoidal section. The prisms had no alliance one with another; they had to depend for their security entirely upon their adherence to the faces of the graded core, so that they could easily be carried off, or become dislocated from natural causes. This system, unlike the first described, did not give a homogeneous envelope with a thickness of its own, and partly independent of the monument which it protected.[222] [221] BÆDEKER, _Egypt_, part i. p. 338 (ed. of 1878). Herodotus (ii. 127) says that the first course of the Great Pyramid was built of a parti-coloured Ethiopian stone (ὑποδείμας τὸν πρῶτον δόμον λίθου Αἰθιοπικοῦ ποικίλου). By Ethiopian stone we must understand, as several illustrations prove, the granite of Syene. The Greek historian seems to have thought that the whole of the first course, throughout the thickness of the pyramid, was of this stone. His mistake was a natural one. In his time the pyramid was in a good state of preservation, and he never thought of asking whether or no the core was of the same material as the outer case. [222] On the other hand, these awkwardly shaped prisms offered less inducement to those who looked upon the pyramids as open quarries than the easily squared blocks of Cheops, while their position in the angles of the internal masonry enabled them to keep their places independently of the lower courses of the casing.--ED. [Illustration: FIG. 155.--The casing of the pyramids; drawn in perspective from the elevation of Perring.] The casing of the Second Pyramid, moreover, does not seem to have been carried out on the same principle from top to bottom. The upper part, which still remains in place, is composed of a hard cement formed of chalk, gypsum, and pieces of burnt brick. They may have wished to obtain the parti-coloured effect of which Philo speaks, by making simultaneous use of granite and concrete, and it is quite possible that yet other materials entered into the composition of the casing.[223] [223] The determination to use a concrete such as that described affords a good reason for the prismatic shape of the granite blocks used in the lower courses. It would evidently be easy enough to cover the pyramid with a coat of cement--working downwards--if its surface did not greatly overpass the salient angles of the steps, while the difficulty would be enormously increased if the coat were to have a considerable thickness of its own independently of the pyramid, like the casing shown in Fig. 155.--ED.