Complex sentence: From 1867 to 1869, he was official photographer on the United States Geological Exploration of the 40th Parallel under Clarence King, which began at Virginia City, Nevada, where he photographed the mines, and worked eastward.
Split into simpler sentences: From 1867 to 1869, he was official photographer on the United States Geological Exploration of the 40th Parallel under Clarence King. The expedition began at Virginia City, Nevada, where he photographed the mines, and worked eastward.