There are, however, sources of error which have not always been sufficiently guarded against in making these estimates. When a boat, composed of several pieces of wood fastened together by pins of the same material, is dug out of a bog, it is inferred that the vessel, and the skeletons and implements found with it, belong to an age when the use of iron was not known to the builders. But this conclusion is not warranted by the simple fact that metals were not employed in its construction; for the Nubians at this day build boats large enough to carry half a dozen persons across the Nile, out of small pieces of acacia wood pinned together entirely with wooden bolts, and large vessels of similar construction are used by the islanders of the Malay archipelago. Nor is the occurrence of flint arrow heads and knives, in conjunction with other evidences of human life, conclusive proof as to the antiquity of the latter. Lyell informs us that some Oriental tribes still continue to use the same stone implements as their ancestors, "after that mighty empires, where the use of metals in the arts was well known, had flourished for three thousand years in their neighborhood;" [Footnote: Antiquity of Man, p. 377.] and the North American Indians now manufacture weapons of stone, and even of glass, chipping them in the latter case out of the bottoms of thick bottles, with great facility. [Footnote: "One of the Indians seated himself near me, and made from a fragment of quartz, with a simple piece of round bone, one end of which was hemispherical, with a small crease in it (as if worn by a thread) the sixteenth of an inch deep, an arrow head which was very sharp and piercing, and such as they use on all their arrows. The skill and rapidity with which it was made, without a blow, but by simply breaking the sharp edges with the creased bone by the strength of his hands--for the crease merely served to prevent the instrument from slipping, affording no leverage--was remarkable."--Reports of Explorations and Surveys for Pacific Railroad, vol. ii., 1855, Lieut. Beckwith'S Report, p. 43. See also American Naturalist for May, 1870, and especially Stevens, Flint Chips, London, 1870, pp. 77 et seq. Mariette Bey lately saw an Egyptian barber shave the head of an Arab with a flint razor.]