550 ME. A. STRAHAN ON OVERTHRUSTS OF [NOV. 1 895, 



pointed out by Mr. Hudleston in 1881 and 1889. 1 In 1876 

 M. Barrois did much towards elucidating the effect of the Isle 

 of Purbeck disturbance by identifying the Upper Chalk zones above 

 and below the fault at Ballard Point. 2 



In 1888 the new Ordnance Survey maps on the scale of 6 inches 

 to the mile having become available, and the geology of the Isle of 

 Wight having been recently re-surveyed on that scale, it was decided 

 by the Director-General of H.M. Geological Survey to continue the 

 re-survey westwards into Dorset. This work was completed, as far 

 as the Secondary rocks south of the Hampshire Basin were con- 

 cerned, in 1890. 3 



During these and the preceding years Mr. Clement Beid had 

 been engaged in working out the structure along the South Coast 

 east of the Isle of Wight, and had succeeded in tracing out a 

 number of post-Cretaceous folds ranging generally east and west, 

 and presumably synchronous with the later group of disturbances 

 about to be described. In addition to these he found at Eastbourne 

 a number of small overthrusts in Chalk, Greensand, and Gault which 

 seem to belong to the same era, though they trend about N.N.E., 

 and therefore cross the others obliquely. These disturbances, which 

 seem not to have been previously noticed, will be described by him 

 in due course. 



The results of our observations on the physical geography of the 

 South Coast have been already brought forward to a certain extent 

 in the 2nd edition of the Memoir on the Isle of Wight. The 

 further evidence obtained in carrying the work westwards will be 

 given in full detail in a Memoir on Purbeck and Portland which is 

 now in preparation, and in which, besides stratigraphical details, 

 much attention has been devoted to the disturbances and to the 

 share that they have taken in producing the existing physical 

 features. In the present paper I propose to give briefly the 

 principal conclusions to which I have been led with regard to the 

 structure of South Dorset. 



II. Grouping and Description of the Disturbances. 



The disturbances can be at once divided into two great groups, 

 the one formed before the deposition of the Upper, but after that of 

 the Lower Cretaceous rocks, and the other being of Tertiary (mainly 

 Miocene) date. To the former belong the anticline of Osmington 

 Mills, the syncline of Upton to the north of it, and the anticline of 

 Chaldon, farther north again, the last-named, however, having been 

 much changed by later movements. In the same group also I place 



1 Proc. Geol. Assoc, vol. vii. p. 176, and vol. xi. p. xlix (Excursion to Wey- 

 mouth). 



2 ' Recherches sur le Terrain Cretace Superieur de l'Angleterre et de TIrlande,' 

 pp. 102, 103, pi. iii. fig. 7. 



3 The 6-inch geological maps of the Dorset Coast and of the Isle of Wight 

 have been mounted as wall-maps and suspended in the Museum of Practical 

 Oeology. 



