The Tertiary physiographic history of Porto Rico was outlined by A. K. Lobeek in 1922.2 His work emphasizes the importance of early and middle Tertiary events, but does not differentiate any landforms of late Tertiary or Quaternary age, except for the relatively recent dissection, marginal drowning, and coastal modification. In 1924 and 1925 the writer studied the physiography of the Virgin Islands, Culebra, and Vieques, which lie immediately east of Porto Rico (figure 1). The land-forms of these islands run those of Porto Rico a close parallel, and in the reports on their physiography 3 the writer has followed, with only minor refinements and revisions, the general chronologic outline established by Lobeck. The development of Porto Rico and the Virgin Islands, as determined in Lobeck’s report and in the author’s studies, is summarized and compared, and the physiographic events are approximately dated in the accompanying . . .