Andesite, however, prevails, and at least three-fourths of the objects are of this material. As most of these rocks in their original distribution are confined to somewhat limited portions of the geological column, some early and others late, it is proposed to inquire whether any of the specimens are of materials later in origin than the strata in which they are said to have been found. Full data are not yet at hand for a satisfactory analysis of this point; but it may be mentioned that andesite specimens are reported from horizons extending all the way from the earliest to the most recent gravels, yet so far as our geologists have gone this rock is not found in the formations of the particular region until toward the close of the Neocene. The objects being generally large, it is not to be supposed for a moment that they could have been brought from a distance. Again, obsidian is, generally speaking, a late product, having its origin in the most recent flows of the sierra, yet we have obsidian implements reported from the gravels of various districts, and in one case, at least, from deposits that must belong very near the initial stages of eruptive activity. This interesting line of research remains to be followed up until definite results are reached; this, however, cannot be done until the geology of the region is more exhaustively studied.
OBVIOUS SOURCES OF ERROR
It was not expected that a short visit to the Auriferous gravel region would lead to a final settlement of the whole question of Auriferous gravel man, for the task is a most difficult one. The recorded observations on which the structure of a Tertiary man is built cannot be made over again or satisfactorily tested, and new observations of a crucial nature must necessarily be of very rare occurrence. However, a reasonably intimate knowledge of the region and its phenomena was gained, and a foundation was laid for future research and for intelligent judgment as to the value of the extensive body of testimony already on record.
