The discovery of such implements was first reported in 1862, and since then a large number have been found, but their authenticity has been hotly contested. The most common were stone mortars very like those of the Indians of the present day, only ruder, and it was objected, first, that they were ground and not chipped, and therefore belonged to the neolithic age; secondly, that they might have slipped down from the surface or been taken down by miners. The difficulty in meeting these objections was that the implements had been found not by scientific men _in situ_, but by ignorant miners, who were too keen in the pursuit of gold to notice the particulars of the find, and only knew that they had picked them out in sorting loads of the gravels, and generally thrown them aside. This, however, had occurred in such a number of instances, over such wide areas, and with such a total absence of any motive on the part of the miners to misrepresent or commit a fraud, that the cumulative evidence became almost irresistible; and we cannot sum it up better than in the words of the latest and best authority, Professor Wright, in an article in the _Century_ of April 1891, which is the more important because only two years previously, in his _Ice Age in North America_, he had still expressed himself as retaining doubts. He says, "But so many of such discoveries have been reported as to make it altogether improbable that the miners were in every case mistaken; and we must conclude that rude stone implements do actually occur in connection with the bones of various extinct animals in the undisturbed strata of the gold-bearing gravel." Fortunately the most important human remains have been found in what may be considered as a test case, where it was physically impossible that they could have been introduced by accident, and where the evidence of a common workman as to the locality of the find is as good as that of a professed geologist. During the deposition of the auriferous gravel on the western flanks of the Sierra there were great outbursts of volcanoes near the summits of that range. Towards their close a vast stream of lava flowed down the shallow valley of the ancient Stanislas river, filling up its channel for forty miles or more, and covering its extensive gravel deposits. The modern Stanislas river has cut across its former bed, and now flows in a gorge from 1200 to 2000 feet deeper than the old valley which was filled up by the lava stream, the surface of which appears as a long flat-topped ridge, known as Table Mountain. In many places the sides of the valley which originally directed the course of the lava have been worn away, so that the walls on either side present a perpendicular face one hundred feet or more in height.