I. I ntroduction The intensity of the earth-movements which have thrown on end and inverted the Secondary rocks of Dorset, and the magnificent dissection of the structures in the sea-cliffs, have attracted attention for many years past. The first to give an accurate account, and to speculate on the causes of the position of the strata, was Webster in 1816; though admirable in his description and in his pictorial representation of the facts, he failed in his endeavour to explain them, and was in this respect improved upon by Conybeare and Phillips in 1822, and by W. B. Clarke in 1837. In 1830 Buckland and De la Beche read their paper on the Geology of Weymouth, etc., and gave an account of the principal faults, but did not correctly distinguish those that were formed before the deposition of the Gault from others that were produced at a later date. In 1848 a cutting made for the Dorchester and Weymouth railway revealed unsuspected complications in the great Ridgeway disturbance, which were described with great accuracy by Weston in 1848 and 1852, while additional light was thrown upon the subject by the publication in 1850 of the Geological Survey map, Sheet 17, in the preparation of which Bristow received great assistance from the Rev. O. Fisher. Further speculations on the age of some of the disturbances were made by Prof. Prestwich in 1875, and the existence of two distinct sets of movements was pointed out by Mr. Hudleston in 1881 and 1889. In