196 THE AURIFEROUS GRAVELS OF THE SIERRA NEVADA. 
intermingled with others of granite and metamorphic rocks. The thickness 
of the mass is nearly 200 feet. Tunnels run under it, and shafts sunk from 
the upper surface — to but little depth, however — seem to have developed 
nothing of value. 
Suppivision IV.—Tue Recion Nortu oF THE SoutH YusA River. 
§ 1. The Divide between the South and Middle Yuba Rivers. 
Tur intention of the writer of this volume was to have the hydraulic 
mining region north of the South Yuba River surveyed with great care; but 
circumstances, to which allusion has already been made in the Preface of 
this work, rendered this impossible. Several of the most important localities 
in the district in question were visited at various times during the continu- 
ance of the Geological Survey, either by the writer himself or by some one 
of his assistants; but most of these examinations, which were not detailed 
and systematic, were made before the hydraulic mines in this extremely in- 
teresting and important region had attained anything like their present 
development and importance. Mr. C. F. Hoffmann, as already mentioned, 
made special surveys in this district, for private parties, which were utilized 
in preparing the large map appended to this volume, and to which reference 
may be made in studying any portion of the gravel region from the Middle 
Yuba, as far south as Michigan Bluffs. 
Parties not connected with the Geological Survey have published impor- 
tant documents in regard to the gravel deposits between the South and Mid- 
dle Yuba, and their work will be made use of to some extent in preparing 
the following pages.* 
* The most important of these is a Report of James D. Hague, entitled ‘‘ The Water and Gravel Min- 
ing Properties belonging to the Eureka Lake and Yuba Canal Company and to M. Zellerback, Esq.,” and 
dated December 22, 1876. Mr. A. J. Bowie, Jr., also gives valuable information, statistical and otherwise, 
in regard to the region in question, in his “ Hydraulic Mining in California,” published in Volume VI. 
of the Transactions of the American Institute of Mining Engineers, dated 1878. In the Fifth, Sixth, and 
Seventh Reports of the United States Commissioner of Mining Statistics, for the years 1873 — 1875, will 
be found various statements of value in regard to the mines of Nevada County, collected and compiled 
by Mr. W. A. Skidmore. It may be stated, however, that it is the intention of the author of this volume 
to procure additional information in regard to the region north of the South Yuba, which will be pub- 
lished either in the form of an appendix or supplementary chapter, as a portion of the present work ; or 
in some other form, so that the important results which have been attained in the development of the 
hydraulic mining interests in Nevada and Sierra counties during the years since the stoppage of the Geo- 
logical Survey shall be placed before the public, in connection with these observations of earlier date. 
