Vol. 51.] OVEETHEtlSTS OF TEETIABY DATE IN DOBSET. 549 



40. On Oveetheusts of Teetiaet Date in Doeset. By A. Steahan,. 

 Esq., M.A., F.G.S. (Bead June 5th, 1895. Communicated 

 by permission of the Director-General of H.M. Geological 

 Survey.) 



[Plates XVII. & XVIIL] 



Contents. 



Page 

 T. Introduction 54S> 



II. Grouping and Description of the Disturbances : 



(1) Post-Cretaceous Disturbances 551 



(«) The Isle of Purbeck Fault and Fold. 



(b) The Ringstead post-Cretaceous Movement. 



(c) The Chaldon and Ridgeway Fold and Overthrust. 



(d) The Litton Cheney Fault. 



(2) Intra-Cretaceous Disturbances 551» 



III. Effects of the Disturbances 56} 



I. Inteodtjction'. 



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, 2 and by W. B. Clarke in 1837. 3 In 1830 Buck- 

 land and De la Beche read their paper on the Geology of Weymouth, 

 etc., 4 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 Bidgeway dis- 

 turbance, which were described with great accuracy by Weston 

 in 1848 and 1852, 5 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 Bev. 0. Fisher. Further speculations on the 

 age of some of the disturbances were made by Prof. Prestwich in 

 1875, 6 and the existence of two distinct sets of movements was. 



1 See Englefieid's ' Isle of Wight,' pp. 164-168, pis. xxvi. & xxvii. 



2 ' Outlines of the Geology of England and Wales,' pp. Ill, 112. 



3 Mag. Nat. Hist, new ser. vol. i. p. 461. 



4 Trans. Geol. Soc. ser. 2, vol. iv. pt. i. (1835) p. 1. 



5 Quart. Journ. Geol. Soc. vol. iv. p. 245, and vol. viii. p. 110. 

 B Ibid. vol. xxxi. p. 43. 



Q. J. G. S. No. 204. 2 r 



