Page:Popular Science Monthly Volume 39.djvu/332

318 stream having access to such, an amount of sedimentary material that for a time it filled up rather than eroded its channel. Apparently the conditions favorable to such effects would be most readily furnished during the Glacial period, when the streams of that region were swollen not only with the increased annual precipitation, but with the melting of the glaciers which doubtless had for a long time occupied the mountains near the head-waters of the Boisé River to the north. Very likely, also, the lava-flows which obstructed the river a few miles above Boisé City turned its course to the southward, so that it may have wandered for some time over the plain in the vicinity of Nampa.

From the erosion of the Boisé River since the outflow of lava it would seem that the time which has elapsed since the volcanic outbursts is closely comparable with that which has passed since the outflow of the lava forming the Table Mountain in Calaveras and Tuolumne Counties, California, under which the famous Calaveras skull was found some years ago. Furthermore, the occurrence of late Pliocene fossils underneath the lava in western Idaho shows that the lava at Nampa is certainly post-Tertiary, so that this discovery of human relics may properly be synchronized with those under Table Mountain in California.

In a visit to Sonora, California, and to Bald Mountain, where the Calaveras skull was discovered, I was so fortunate also myself as to run upon evidence of a previously unreported instance of the discovery of a stone mortar under Table Mountain. The mortar was found in October, 1887, by Mr. C. McTarnahan, the assistant surveyor of Tuolumne County. It was lying in the gravel reached by the Empire Tunnel, and about a mile west of the Valentine shaft where Dr. Snell found a similar relic. This tunnel had been excavated seven hundred and fifty-eight feet before reaching the gravel, and the mortar was found one hundred and seventy-five feet in a horizontal line from the edge of the Table Mountain basalt, and about one hundred feet below the surface. The object was taken out and laid beside the mouth of the tunnel, and was given to Mrs. M. J. Darwin, of Santa Rosa, California, who has since given it to me. The mortar is made from a small bowlder of some eruptive rock, and is six and a half inches through; the hollow being about three and a half inches in diameter, and about three inches deep.

At the annual meeting of the Geological Society of America, in January, 1891, a similar mortar was reported by Mr. George F. Becker, of the United States Geological Survey, as found under Table Mountain, about five miles south of the Empire mine, near Rawhide Gulch. Mr. J. H. Neale, a well-known mining superintendent, made his affidavit that he took this with some other objects of human manufacture from undisturbed gravel underneath