During recent years, geologists have given increased attention to the internal structures of igneous rocks. Studies have been greatly stimulated by the work of Hans Cloos and his school of geologists in Europe. Because of his acquaintance with this work, the writer was asked by Dr. Frank F. Grout, chairman of the Batholith Committee of the National Research Council, to summarize the present state of knowledge in this field and thus make it available to American geologists. This presentation outlines in detail the methods used, and some of the results attained, in Europe and in the United States. No attempt has been made to cover the entire literature on structural geology of igneous rocks, but the more important contributions are included. 1 The principal value of the researches of Dr. Cloos lies in the fact that he has shown that there is a dependable relation between the movements of the individual blocks in the earth’s crust and their internal structure. Although many other investigators—Van Hise, Leith, Mead, Bowie, Bucher, Chamberlin, Heim, Sander, Becke, Gutenberg, and others—have developed petrographic, microscopic, and geophysical methods by which information is gained as to the dynamic history and the physical behavior of rocks in the earth’s crust, the field of dynamic geology is so broad that these lines of work supplement, rather than overlap, one another. As each method develops, more and more points of contact are established, and it is practically certain that the general structural problem, which now is approached by one