PREFACE. 
Tue title of the work here presented to the public seems to explain its object 
with sufficient clearness; but a few words in regard to the character and source of 
the materials which have been used in its preparation are perhaps desirable. The 
stoppage of the State Geological Survey by the Legislature of California, in 1874, 
left me in possession of considerable information, both of my own collecting and 
that resulting from the labor of others; and, having obtained from the Regents of 
the State University authority to continue the publications of the Survey at my own 
expense, or on my own responsibility, I have already added several volumes to 
the list of those which had been issued at the time of the closing of the work by the 
Legislature. In the publication of the Botany, I have had assistance from a few 
generous and public-spirited residents of San Francisco. The present volume, although 
really a continuation of the work of the Geological Survey, is issued in quarto form, 
in consequence of an arrangement with the curator of the Museum of Comparative 
Zoology, at Cambridge, whereby that institution assumes a portion of the expense of 
publication, receiving a certain number of copies of the work for distribution among 
the societies and individuals on its exchange list. One volume, certainly, and probably 
others, will follow this, their publication having been provided for in a similar way. 
Those volumes which belong to divisions of the work which have been already com- 
menced —as, for instance, the continuation of the Ornithology — will appear in royal 
octavo form, to match the already published volumes of the regular series of the Survey ; 
the others will be of quarto size. 
At the time of the discontinuance of the California Survey, the publication of a volume 
having as its subject the auriferous gravels had been for some time in contemplation, 
but its appearance had been delayed by a series of misfortunes, of which it is not neces- 
sary here to speak. Nothing, therefore, in this particular division of the geological work, 
was quite ready for the press at the time of the abandonment of the Survey by the 
Legislature. When, later, after considerable hesitation, I had concluded to resume the 
work of publication, the gravel volume was one of those with which I was most anxious 
to proceed. In view, however, of the great importance of the hypsometrical element in 
the survey of the gravel region, it appeared to me extremely desirable that our baromet- 
