From Introduction: "This bulletin is the fourth in the series of petroleum bibliographies being published by the Bureau of Mines, the preceding, Bulletins 149, 165, and 180, being complications for the years 1915, 1916, and 1917, respectively. In this bulletin the same general plan has been followed in arranging the references, but a few changes and additions have been made in classification, the more important changes being in the classes "Geology and Origin" and "Refining and Refineries." During the year German periodicals were not available, and as in the bibliography for 1917, references to articles in foreign journals that were not available have been taken from other sources and credit given to those sources."