GEOLOGY. 351 



gravel in which the skull was found, nine feet. Some attempts 

 have been made to discredit this discovery ; but those who 

 have made the most careful investigation of the facts, and 

 whose opinions are entitled to the most weight, accept it. 



Amos Bowman, of the State Geological Survey, thus defines 

 certain eras in the geological history of California : 



1st. The Pliocene, or ancient eroding period, during which 

 these deep '' dead " river channels were cut into the " bed- 

 rock." 



2d. These Pliocene channels filling up with gravel or the 

 choking or damming period. 



3d. The active volcanic period of the Sierra, when the 

 gravels were capped with lava and volcanic ashes. 



4th. The cold, or glacial period, when the mountain slopes 

 were covered with living, moving glaciers. 



5th. The modern erosive, or recent period, during which 

 the present river channels were formed, crossing the old chan- 

 nels at various angles. 



Dr. James Blake, of San Francisco, reported to the Acad- 

 emy of Sciences, in the beginning of 1873, the discovery of 

 some' artificial stone ornaments, in a Pliocene formation, near 

 San Francisco, indicating the existence of men here in the 

 Pliocene era. 



Stone mortars, pestles, and arrow-heads, have been found, 

 according to report, in Pliocene gravel, at Murphy's Camp, 

 Shaw's Flat, Columbia, Springfield, Tuolumne Table Mountain , 

 Kincaid Flat, French Bar and Cottonwood, in Siskiyou 

 County, Spanish Flat and Soapweed, in El Dorado County. 



In May, 1859, an Indian arrow-head was found, eighty feet 

 below the surface of the earth, at Buckeye Hill, Nevada 

 County. About the same time, another arrow-head was 

 found three feet deep in undisturbed alluvium, near Freeman's 

 Crossing, in the same county. 



In April, 1859, the skeleton of a man was found sixteen 

 feet deep at Tehachepe, in Los Angeles County. 



