INTRODUCTION The collection which is the basis for the present report was made by Señor José Ramón Guiñazú, of the Dirección General de Minas y Geología de la República Argentina, and I am greatly indebted to him and to that organization for the privilege of studying this most interesting fossil flora. Many others, in addition to previous writers, have been of material assistance, and I record my grateful thanks to Dr. George Gaylord Simpson, of the American Museum of Natural History, who, as leader of the Scarritt Patagonian expeditions, brought this magnificent collection to my attention; to Señor I. Rafael Cordini, of Buenos Aires, for a most excellent collection of recent plants from the Lago Nahuel Huapi region, which after having been used by me in these studies I presented to the United States National Herbarium; to Dr. Frank M. Carpenter, of the Museum of Comparative Zoology, who has examined the insect remains associated with the fossil plants; to Dr. Bailey Willis, for his generosity in placing his Patagonian photographs at my disposal; and to the Geological Society of America, for a grant to defray the cost of photographing the fossil specimens. My task has been considerably lightened by the available information on the geography of the region contained in the publication that resulted from the survey made in 1911–14 by the Ministry of Public Works and directed by Bailey Willis (1914), covering the region from the port of San Antonio, on the Atlantic coast, past the Rio Pichileufu . . .