ACRIFEROUS GRAVELS— INDEPENDENCE HILL FLORA 885 

 THE PAL.EOBOTANICAL EVIDENCE OF AGE. 



Almost the only means of determining the age of the 

 auriferous gravels is afforded by the numerous plant remains 

 which they sometimes contain. Remains of vertebrate ani- 

 mals have been found, but as Mr. H. W. Turner has pointed 

 out, there is much doubt as to whether certain of the occur- 

 rences described by Whitney really belong to the auriferous 

 gravels, so that this line of evidence does not at present lead 

 to any satisfactory conclusions. 



The age of the deep gravels. — No fossils of any kind have, 

 as far as I am aware, been found in these beds. The coarse 

 character of the gravels is largely responsible for this. As the 

 bench gravels will be shown in the following pages to be of 

 Miocene Age, the deep gravels are manifestly older, and some 

 of them may be Eocene, but hardly older. 



The age of the bench gravels. — The best and most extensive 

 collections come from the upper part of the bench gravels, or 

 from the very lowest part of the rhyolitic tuffs. 



The Chalk Bbiffs locality. — The largest collection examined 

 bv Professor Lesquereux was gathered by Mr. C. U. Voy, at 

 Chalk Bluffs, near You Bet, Nevada county, and it therefore 

 becomes desirable to indicate its exact horizon. You Bet is 

 situated on the main channel of the Neocene South Fork of the 

 Yuba, and the leaves occur at an elevation of about 3000 feet. 

 The stratigraphic relations are extremely similar to those of 

 Iowa Hill (Fig. i) a little further south on the drainage of the 

 old American River. There are the same deep gravels and 

 heavy spreading bench gravels, capped by rhyolitic tuffs and 

 andesitic tuffaceous breccia. 



A bluff of the volcanic beds has been exposed by the 

 hvdraulic mining operations, and owes its name to the brilliant 

 white color of the rhyolite. The exact locality from which the 

 leaves came is a matter of some doubt, and is now covered by 

 sliding debris. I was told by residents that they occurred at a 

 place near the top of the bench gravels just below the rhyolitic 



