28 Prof. BucKLAND and Mr. De la Beche on the 



Oxford Clay. 



The Oxford clay in this district attains a thickness of about 300 feet^ and 

 reposes immediately, and with conformable dip, on the central arch of corn- 

 brash and forest marble; on the soutli side of this arch it forms a belt of low 

 ground about a mile broad, extending from Melcombe Regis and Weymouth 

 Bay to East Fleet. It winds round the eastern extremity of the forest marble 

 at the Barracks, and the marsh of Lodmoor, and thence extends westward 

 along the north side of the central arch, forming a valley of less than half a 

 mile in width, to the Decoy and Swanery at Abbotsbury, where it terminates 

 in the Chesil Bank. Its eastern termination is seen in the lowest strata 

 at the base of Jordon Hill and Ham Cliff, about three miles north-east of 

 Weymouth*. 



A distinct section of this stratum occurs in a low cliff under the Barracks 

 at Radipole, where it dips south at about 20°, and where, till lately, it pre- 

 sented, at low water, a pavement of large and beautiful Septaria, known pro- 

 vincially by the name of Turtle Stones. The veins of these Septaria are filled 

 with yellow semi-transparent calcareous spar, often passing into a rich deep 

 brown colour : their beauty, when polished, has, within these few years, 

 caused the greater part of them to be taken up and manufactured into slabs 

 and tables. 



In this same section at Radipole Barracks large Ammonites and other 

 shells abound. The most characteristic shell is the Gryphcca dilatata, which 

 is as universally abundant in the Oxford clay throughout this district, as the 

 deltoid oyster is in the Kimmeridge clay. In consequence of the want of 

 sections, we have not attempted to trace the minute details of this clay in its 

 windings through the Vale of Bredy, in which it forms a marginal band round 

 the eastern and northern frontier of the forest marble. On the opposite coast 

 of Normandy this same Oxford clay occurs at Vaches Noires, and abounds 

 with the GryphcBa dilatata, and many other fossil shells. 



Forest Marble. 



The formations, which, for convenience of description we have united 

 under the name of Forest marble, and which are the lowest that occur in the 

 Vale of Weymouth, admit of a three-fold division ; — 1. Cornbrash; 2. Forest 

 marble ; 3. Clay, and marlstone. 



1, The uppermost region, or cornbrash, is composed of a loose rubbly 

 limestone, alternating with thin beds of clay and marl, and is so absolutely 



* See Plate II. fig. 8. 



