Vol. 51.] TERTIARY DATE IN DORSET. 553 



at a little over 100 feet below sea-level, indicating a region of com- 

 pression at that depth. Upwards, on the other hand, the two 

 formations diverge equally rapidly, indicating a tendency to gape. 

 The cliff intersects parts of both regions, and shows the behaviour 

 of the more yielding strata under the peculiar circumstances. The 

 beds affected are the Upper and Middle Purbeck from the Cinder 

 Bed upwards ; they consist of shales and clays with numerous rock- 

 bands, most of which are easily recognized. At the foot of the cliff 

 these harder beds are crowded together, the softer partings having 

 been squeezed out, but at the same time a very sharp little anticline 

 in the Cinder Bed just shows above the foreshore. Traced upwards 

 in the cliff this little fold gains size in every succeeding zone, until 

 In the Unio-be&s it becomes a large inverted A- The result of these 

 movements has been that strata originally 200 feet wide have been 

 packed away in a width of about 100, the loss of space in the 

 direction of the bedding-planes being compensated for by a gain in 

 a direction at right angles to them. In this case the strata of the 

 A-fold have moved not less than 100 feet relatively to the Lower 

 Purbeck, not along any one fracture, but by a gliding-movement 

 along the bedding-planes. The well-known contortions in the 

 Upper and Middle Purbeck of Stare Cove seem to have originated in 

 <the same way. 



The inversion of the Wealden Beds diminishes northwards through 

 verticality to a normal dip of 50° and 45° in the upper part of the 

 formation. The Upper Chalk, however, must be more highly inclined 

 than this, for both east and west of the line of section it is either 

 vertical or inverted. Hence follows the irregularity of the curve of 

 the monocline shown in the section. Of the Isle of Purbeck fault 

 little is known here, except that nearly horizontal beds (Tertiary 

 and Chalk) occur in close proximity to vertical Upper Chalk. 



At Dungy Head, which lies about midway between Sections 2 

 and 3, the cliff intersects the region of compression which is partly 

 seen at Lulworth. The Portland Stone and Lower Purbeck dip 

 steeply towards Middle Purbeck rocks in the upper part of the cliff, 

 but at the foot of the cliff the Lower Purbeck strata are squeezed 

 •out to a mere line, so that the Cinder Bed comes into contact with 

 the Portland Stone. Between the Middle Purbeck and the Wealden 

 there is also a squeeze-out, for the Upper Purbeck and an unknown 

 amount of the Wealden have disappeared. Some of the Gault has 

 suffered in a similar manner. 



Section 3 (PI. XVIII.) is drawn through the eastern end of the 

 Chaldon anticline and thence southwards through the Durdle 

 promontory, where the Isle of Purbeck fold attains its greatest 

 intensity. The strata have not only undergone wholesale inversion 

 and compression, but have been horizontally fractured and shifted 

 in a manner not seen elsewhere. 



The Portland Stone and Lower Purbeck stand nearly vertically ; 

 but most of the Middle Purbeck (excepting the Cinder Bed), all the 

 Upper Purbeck, and some of the Wealden have been squeezed out. 



