In 1867, King was named U.S. Geologist of the Geological Exploration of the Fortieth Parallel, commonly known as the Fortieth Parallel Survey, a position for which he strongly lobbied. King spent six years in the field exploring areas from Wyoming to the border of California. During that time he also published his famous Mountaineering in the Sierra Nevada (1872). After the completion of the field work, in 1878 King published Systematic Geology. In this work he narrated the geological history of the West as a mixture of uniformitarianism and catastrophism. 
While King was finishing on his 40th Parallel Survey, the western US was abuzz with news of a secret diamond deposit. King and some of his crew tracked down the secret location in northwest Colorado, and exposed it as a fraud, now known as the Diamond hoax of 1872. His exposure of the diamond fraud helped build his heroic national reputation.
While conducting field work for the Survey, King met and became friends with Henry Brooks Adams. Their friendship lasted for the rest of King's life, and he is often mentioned in reverent and adoring terms by Adams in the autobiographical The Education of Henry Adams (1907). From descriptions of King appearing in both Adams and Sandweiss, it is clear that King was unusually intelligent, witty, charming and magnetic – and a one-of-a-kind conversationalist. People in general were drawn to him.
In 1879, the US Congress consolidated the number of geological surveys exploring the American West and created the United States Geological Survey. King was chosen as ts first director; however, he served for only twenty months.
King spent his last thirteen years leading a double life. In 1887 or 1888, he met and fell in love with Ada Copeland, an African-American nursemaid (and former slave) from Georgia, who had moved to New York City in the mid-1880s. As miscegenation was strongly discouraged in the nineteenth century (and illegal in many places), King hid his identity from Copeland. Despite his blue eyes and fair complexion, King convinced Copeland that he was an African-American Pullman porter named James Todd. The two fell in love and entered into a common law marriage in 1888. Throughout the marriage, King never revealed his true identity to Ada, pretending to be Todd, a black railroad worker, when at home, and continuing to work as King, a white geologist, when in the field. Their union produced five children, four of whom survived to adulthood. Their two daughters married white men; their two sons served classified as blacks during World War I. King finally revealed his true identity to Copeland in a letter he wrote to her while on his deathbed in Arizona.
^Geological Survey of California, J.D. Whitney (1865). "Geology, volume 1", Sherman & Co, Philadelphia
^King, Clarence. 1872. Copy of official letter, addressed November 11th, 1872, to the Board of Directors of the San Francisco and New York Mining and Commercial Company, "... discovering the new diamond fields to be a fraud." San Francisco and New York Mining and Commercial Company. [San Francisco? 1872]. pp. 12.
^Sandweiss, Martha A. (2009). Passing Strange: A Gilded Age Tale of Love and Deception Across the Color Line. ISBN1-59420-200-1.
Works about King
Aaron Sachs, The Humboldt Current: Nineteenth Century Exploration and the Roots of American Environmentalism, (Viking, 2006), King, one of four Americans on whom the author focuses, was influenced by Alexander von Humboldt.
Thurman Wilkins and Caroline Lawson Hinkley (1988). Clarence King: A Biography, University of New Mexico Press, 1988 revised edition, softcover, ISBN 0-8263-1085-0
Robert Wilson (2006). The Explorer King : Adventure, Science, and the Great Diamond Hoax – Clarence King in the Old West, Scribner, ISBN 0-7432-6025-2
Works by King
King, Clarence. "Catastrophism and Evolution", The American Naturalist, Vol. 11, No. 8. (August 1877), pp. 449–470.
King, Clarence. 1872. Copy of official letter, addressed November 11, 1872, to the Board of Directors of the San Francisco and New York Mining and Commercial Company ... discovering the new diamond fields to be a fraud." San Francisco and New York Mining and Commercial Company. [San Francisco? 1872]. 12 pages. 24 cm. Held in the USGS Library.
King, Clarence. 1881. Records of the geological exploration of the fortieth parallel ("King Survey"), 1867–81. Geological Survey (U.S.) 1842–1901. National Archives (U.S.), Geographical and Geological Survey of the Rocky Mountain Region (U.S.). File microcopies of records in the National Archives; no. M622. 3 volumes. Notes: "Part of the records in the National Archives designated as Record Group 57, Records of the Geological Survey" – p. iii. Microfilm. Washington: National Archives, 1965. 3 microfilm reels; 35 mm.
King, Clarence. 187?. Atlas accompanying Volume III on Mining industry / United States Geological Exploration of the Fortieth Parallel. Clarence King, Geologist in charge. United States Geological Exploration of the Fortieth Parallel. New York: J. Bien, [187–?]. 16 leaves of maps (part col.); 49 x 64 cm. Notes: Includes 14 numbered plates.
Report of the geological exploration of the fortieth parallel. by United States. Geological Exploration of the Fortieth Parallel, King, Clarence, 1842–1901, Hague, A. (Arnold), 1840–1917, Emmons, Samuel Franklin, 1841–1911, Hague, James D. (James Duncan), 1836–1908, Meek, F. B. (Fielding Bradford), 1817–1876, Hall, James, 1811–1898, Whitfield, Robert Parr, 1828–1910. Ridgway, Robert, 1850–1929., Watson, Sereno, 1826–1829, Eaton, Daniel Cady, 1834–1895, Bien, Julius, 1826–1909. [Washington : G.P.O., 1870–80]. Imprint: 7 v. : ill., maps and atlas. 2v. 50 x 64 cm. and 83 x 63 cm. Notes: Each volume has special t.p. Atlases have titles as follows: "Atlas accompanying volume III. Mining industry ... New York, Engraved and printed by Julius Bien" and "Geological and topographical atlas ... 1876. Julius Bien, lith." I. (1878) Systematic geology, by Clarence King. – II. (1877) Descriptive geology, by Arnold Hague and S.F. Emmons. – III. (1870) Mining industry, by James D. Hague, with geological contributions by Clarence King. – IV. (1877) pt. I. Palaeontology, by F.B. Meek. pt. II. Palaeontology, by James Hall and R.P. Whitfield. pt. III. Ornithology, by Robert Ridgway. – V. (1871) Botany. By Sereno Watson, aided by Prof. Daniel C. Eaton, and others. – VI. (1876) Microscopical petrography, by Ferdinand Zirkel. – VII. (1880) Odonthornithes:a monograph on the extinct toothed birds of North America ... by Othniel Charles Marsh.
Report of the Public Lands Commission, by United States. Public Lands Commission (1903–1905), Williamson, James Alexander, 1829–1902, King, Clarence, 1842–1901, Britton, Alexander Thompson, 1835–1899, Donaldson, Thomas, 1843–1898, Powell, John Wesley, 1834–1902. Washington, Govt. Print. Off., 1880. Imprint: xlvii, , xlix–c p., 1 l., 690 p. fold. plates (1 col.) fold. maps, diagrs. 24 cm. Notes: Preliminary report of the Public lands commission (p. [v]–xlvii) signed: J.A. Williamson, Clarence King, A.T. Britton, Thomas Donaldson, J.W. Powell.
Clarence King memoirs. The helmet of Mambrino, by Hague, James D. (James Duncan), 1836–1908, King, Clarence, 1842–1901. Century Association (New York, N.Y.). King Memorial Committee. New York: Published for the King Memorial Committee of the Century Association by G.P. Putnam's Sons, 1904. Imprint: vii, 429 p.,  leaves of plates : facsim., 9 ports.; 22 cm. Notes: Committee chairman: James D. Hague. "The helmet of Mambrino" was first published in the Century magazine, May 1886. Includes bibliographical references (p. 372–374) and index. The helmet of Mambrino / Clarence King – Don Horatio / James D. Hague – Clarence King / John Hay – Meetings with King / William Dean Howells – King / Henry Adams – Clarence King / John LaFarge – King, "The frolic and the gentle" / Edmund Clarence Stedman – King at the Century / William Crary Brownell – Century necrological note / Edward Cary – King's "Mountaineering" / Edward Cary – Clarence King, geologist / Samuel Franklin Emmons – Clarence King's school-days / Daniel C. Gilman – Biographical notice / Rossiter W. Raymond – Memoriabilia / James D. Hague.
King, Clarence. 1881, 1882. Statistics of the production of the precious metals in the United States. King, Clarence, 1842–1901., United States. Census Office. Washington: Gov't Print. Off., 1881. 94 p.: 6 col. pl. (incl. diagr.) ; 30 cm. Notes: At head of title: Department of the Interior. Tenth census of the United States. Francis A. Walker, superintendent. Also: Extract from the Annual report of the director of the U.S. Geological Survey, [2d] 1880–81. Cover title.
King, Clarence. 187?. Atlas accompanying volume III on Mining industry / United States Geological Exploration of the Fortieth Parallel; Clarence King, Geologist in charge. United States Geological Exploration of the Fortieth Parallel., New York : J. Bien, [187–?]. 16 leaves of maps (part col.) ; 49 x 64 cm. Notes: Includes 14 numbered plates.
King, Clarence. 1881. Records of the geological exploration of the fortieth parallel ("King Survey"), 1867–81 by Geological Survey (U.S.), 1842–1901., National Archives (U.S.), Geographical and Geological Survey of the Rocky Mountain Region (U.S.). File microcopies of records in the National Archives; no. M622. 3 volumes. Notes: "Part of the records in the National Archives designated as Record Group 57, Records of the Geological Survey" – p. iii. Microfilm. Washington: National Archives, 1965. 3 microfilm reels; 35 mm.