Letter of transmittal: I have the honor to transmit herewith a report on the geology of the island of Nantucket. This report embodies a portion of the work which I have done in connection with the general study of the Atlantic coast line. Although part of a much more extended inquiry, it seems well to publish the results contained in this memoir in advance of my general report for the reason that the region constitutes a distinctly bounded field and also because the problems considered in this report have an important bearing on work which is in the bands of other divisions of the Survey.