It is difficult at present to test such chance evidence accurately. The discovery of the palæolithic implements of Europe had been recorded upwards of half a century before their true significance was recognised; whereas the American explorer is on the look-out for similar disclosures, and evinces at times a feeling as though the honour of his country is imperilled if he fail. It will be seen, moreover, from the narrative of a subsequent chapter, that the abundance of flint and stone implements in the virgin soil of the New World is almost marvellous. The discovery, therefore, of stray specimens in modern river-gravels, the washings of gold-drift, or in any excavations liable to be affected by surface admixtures, must be viewed with the utmost caution. Several flint implements from the auriferous gravel of California were produced at the Paris Exposition of 1855. According to the geological survey of Illinois, for 1866, the bones of the mastodon and other fossil mammals have been found in a bed of “local drift” near Alton, underlying the Loess; and at the same depth stone axes and flint spear-heads were obtained.[25] But such disclosures of worked flints or polished implements of stone are cast into the shade by the reputed discovery of human remains in the auriferous drift of California. In 1857 Dr. C. F. Winslow produced a fragment of a human skull found eighteen feet below the surface, in the “pay drift,” at Table Mountain, in connection with the bones of the mastodon and fossil elephant. A later disclosure brought to light a complete human skull, reported to have been discovered in auriferous gravel, underlying five successive lava formations. Professor Whitney, after satisfying himself of the genuineness of the discovery, produced the skull at the Chicago meeting of the American Association for the Advancement of Science, in 1869, to the manifest delight of some who were prepared at once to relegate American man to a remoter epoch than the Flint-folk of the Abbeville and Amiens gravel drift. More recently a highly polished plummet of syenite, in the form of a double cone perforated at one end, was produced before the Chicago Academy of Sciences, as an implement found at a depth of thirty feet, in the drift-gravel of San Joaquin, California, by some workmen engaged in digging a well. In this case also Professor Whitney appears to have had no hesitation in assigning it to the age of the fossil elephant and mastodon. It does not seem to have been recognised how much more probable it is that a highly finished stone implement like the San Joaquin plummet should fall from the surface, in the process of excavation, and so be perhaps no older than the era of the Mexican conquest, than that it is a choice specimen of post-pleiocene art.