In an introductory section of the paper, a sketch is given of the literature of the subject, and reference is made to the labours of Jameson, Macculloch, Berger, Boue, Oyenhausen, Von Dechen, Necker, Zirkel, Judd, and other observers. The author then mentions the progress of his own investigations, which were begun some thirty years ago, and of which the first published part appeared in the Transactions of the Society for 1861. A journey into Western America in 1879 gave him new insight into the problems presented by the youngest volcanic rocks of Britain. Since that time he has devoted every available interval to the prosecution of this research, and he now offers the completed results to the Society, which encouraged him by printing his earliest communication on the subject.