150 THE GEOLOGY OF THE PURBECK HILLS. 



Geological Survey, Isle of Purbeck," and also in his " Guide 

 to the Geological Model of the Isle of Purbeck." Dr. Strahan 

 has not, however, given diagrams to illustrate the various 

 steps in the process, and hence his description is by no means 

 easy to follow. His account also depends on a certain con- 

 tingency which further study may prove to be incorrect, 

 and hi that case the method of construction must have been 

 materially different. In Plate II., Figs. 1 and 2, and Plate 

 III., Fig. 1, I have drawn diagrams illustrating the several 

 steps in the process as described by Dr. Strahan, and in 

 Plate III., Figs. 2 and 3, two diagrams are given illustrating 

 the alternative explanation, which may yet prove to be the 

 more correct. I now give a description of these diagrams 

 and the geological processes they are designed to illustrate. 



Until near the close of the Oligocene period the strata 

 of this district remaineed almost in the horizontal position 

 in which they had been deposited ; but at this period earth 

 shrinkage commenced to upheave the great anticline indi- 

 cated in outline in Plate I., Fig. 1. No fracture commenced 

 until this had proceeded probably to its highest point, which 

 was then as now in the Purbeck area, and still exists 

 in the chalk monocline of the Isle of Wight where it has 

 remained unfractured until now. The condition would 

 then have been as indicated in Plate II., Fig. 1, which shows 

 on a larger scale the monoclinal edge of the great anticline 

 with which we have now to deal. Tertiary beds, since denuded 

 off, must then have overlain the chalk, and probably these 

 were in places covered by Oligocene beds, a remnant of 

 which is still found (as described by the late Mr. Hudleston) 

 capping Creech Barrow. The movement continuing, a 

 fracture began to form, and the question arises, where did this 

 first occur ? The researches of the French geologist, C. 

 Barrois * (" Terrain Cretace Superieur," 1896), confirmed 



* Dr. Barrois supposed that the northern beds belonged to the Marsupite.s 

 zone, but this has since been disproved. (Dr. Rowe, op. ct., p. 36.) 



