Frobenius states, regarding one of these subterranean chambers which he explored, that it contained a dome which was paneled and strengthened with wood from the borassus palm and the whole plastered with a sort of prepared clay.[17] Frobenius also believes that the external parts of the tombs, that is, the mound proper--was made layer by layer. Each layer of clay was first thoroughly worked, moulded, and baked. This process was repeated time and time again, until the mound was completed. The veteran Egyptologist, Flinders Petrie, in the great mass of evidence adduced by him to show the African origin of the spirit and substratum of early dynastic Egyptian culture, points out that there is a very close connection between the subterranean structures of these tombs and many of those of the Egyptian pyramids, the inference being that the idea of the pyramids very probably had its origin in Central Africa. As interesting and important as are these structures in this connection, they, like those previously mentioned and those yet to be described, are of interest in another direction; they bespeak the sometime existence here of a mighty people with a glorious past, now lying sleeping within the bosom of the earth, the silent witnesses of a world that has perished. Beginning about three hundred years ago, and going back to an unknown period, it is evident from the above comments and extracts that the cultural life of the Negro on the West Coast of Africa, especially from the point of view of his architectural and tomb-building proclivities, was of a much higher type than anything he has produced since his contact with the European during the last four hundred years. The quality and quantity of work accomplished by these ancient black builders is especially notable when it is remembered that the type of material which they were forced to use, and the climatic conditions surrounding them, were of a most discouraging sort. The manner in which these very serious difficulties were overcome is itself a durable testimony of the ingenuity and resourcefulness of the African builder and craftsman of earlier days. One can hardly avoid the speculation of what might have been the nature of their accomplishments, had they been provided with a more suitable and durable building material.