The formation there is a lacustrine deposit of great thickness, belonging to the upper Miocene, and abounding in flint. Here, during the course of twenty years, M. Ribeiro discovered, but mostly upon the surface, a large number of flakes of flint and quartzite. After much debate in regard to them, ninety-five of them were finally sent by him to Paris, in 1878, and placed in the archæological department of the great exposition. There they were to be submitted to the judgment of the assembled prehistoric archæologists of all nationalities, many of whom, including the writer, availed themselves of the opportunity of carefully studying them. The judgment of Mortillet is that twenty-two specimens exhibited unmistakable traces of intentional chipping, in which opinion I entirely concur. Only nine, however, were represented as coming from the Miocene, some of which showed on their surface an incrustation of grit, which was claimed as proof of their origin. But the opinion was freely expressed that, even if they really came from the Miocene deposits, they might have penetrated into them from the surface, through cracks, and thus have become so incrusted. It was accordingly resolved to hold the next international congress of prehistoric archæologists at Lisbon, in 1880, mainly for the purpose of settling this question, if possible, by an investigation conducted upon the spot. In the course of a visit made at that time to Otta, several artificial specimens were found on the surface by different searchers, but Professor Bellucci, of Perugia, was fortunate enough to discover a flint flake _in situ_, still so closely imbedded in the deposit that it required to be detached by a hammer. There is no question that this object was actually found in a Miocene deposit, but unfortunately it belongs to the doubtful category of external flakes, which, although they exhibit the "bulb of percussion," have no other sure indication that they are the work of man.[FI] As such bulbs can be produced by natural causes, some stronger proof than this of the existence of Tertiary man is demanded. [Footnote FI: It has been figured by Bellucci, _Archivio per l'Anthropologia e la Etnologia di Firenze_, tome xi, p. 12, tav. iv, fig. 2. To me it possesses no value as evidence.]