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 Mr. King chose as his geological assistants the brothers J. D. and Arnold Hague, and S. F. Emmons, all of whom had received good training in the geological survey of California, and proved competent and congenial co-laborers with him. Specialists were also engaged to study the materials collected in other departments of natural science; the fossils being placed in the hands of Mr. F. B. Meek, and, after his death, in those of Professor James Hall and Mr. R. P. Whitfield. The botanist of the survey was Mr. Sereno Watson, and the ornithologist Mr. Robert Ridgway, while a special investigation of the volcanic rocks collected was made by Professor Ferdinand Zirkel, the distinguished lithologist of Leipsic. The topographical work of the survey was, during the greater part of its continuance, in charge of Mr. J. T. Gardner, now director of the Topographical Survey of the State of New York.

The report of the "Survey of the Fortieth Parallel," as now published, consists of six volumes quarto. Of these the first issued (1870) was Vol. III. of the series, entitled "Mining Industry," and devoted to a description and discussion of the important mines and mining districts embraced in the territory studied. About half of the volume is taken up with a detailed description of the Comstock lode—the most productive deposit of gold and silver yet discovered—and of the methods of exploitation followed in the different mines located upon it, by Mr. King and J. D. Hague. It also contains chapters by Mr. Arnold Hague on the "Chemistry of the Washoe Silver Process," and on the "Geology of the White Pine Mining District"; the "Geology of the Toyabe Range," by S. F. Emmons; and on the "Geological Distribution of Mining Districts," and the "Green River Coal Basin," by Mr. King. This volume is accompanied by an atlas of maps, plans, and sections, which at the time of its appearance was by far the most beautiful work of its kind published in this country.

It is greatly to be regretted that the review of the mining industry of the country bordering the Pacific Railroad, so admirably begun in this volume, could not have been continued through the life of the survey, and have been presented to the public at its close. Since the publication of this discussion of the Comstock lode, the most important chapters in its history have been enacted, and it is a pity they should not also have been written. When Mr. King closed his examination the workings had reached a depth of a little more than 1,000 feet, and the maximum temperature, (of water at bottom) was about 108° Fahr.; and now the depth of 2,500 feet has been reached, the temperature of the water at the bottom is 160°, and facts have been obtained which indicate that the limits of the successful working of the lode will be determined by temperature and these limits soon be reached.

The Sutro Tunnel has also been constructed and all its geological revelations made since the appearance of Vol. III. The great bubbles of the White Pine and the Emma mines have since swollen and burst,