•562 OVERTHRUSTS OF TERTIARY" DATE IN DORSET. [N~OV. 1 895, 



apparent barrier of Chalk hills between the sources of the streams 

 and their destination. 



The Bidgeway axis of upheaval has played the same part in 

 determining the drainage-lines. The crest of the anticline ran close 

 to the line of fault as it now appears at the surface, and northwards 

 from it the streams ran to join the Frome, while south of it they 

 made their way to the English Channel, ignoring the folds in the 

 Oolitic rocks. The Bidgewa} r line of Downs, from a topographical 

 point of view, compares with the Purbeck or Isle of Wight Chalk- 

 hills. Yet, for the reasons stated, the one is a water-parting, the 

 others are not. 



EXPLANATION OF PLATES XVII. & XVIII. 



Plate XVII. 



Map showing the intra-Cretaceous and post-Cretaceous Disturbances near 

 Weymouth. (Scale : 1 inch=l mile.) 



Plate XVIII. 



Sections through and across the Isle of Purbeck Disturbance, and across the 

 Ohaldon and Bidgeway Disturbances. 



7 = Wealden Beds. 



(7'=Wealden Shales.) 

 (7 = Hastings Beds.) 



6=Purbeck. 



13= Tertiary Beds. 

 12=Upper Chalk. 



(12' =Belemnitella-zones.) 

 ll=Middle Chalk. 

 10=Lower Chalk. 

 9= Upper Greensand and Gault. 

 8 = Lower Greensand. 



(8' = Variegated Sands.) 

 (8 =Atherfield Clay.) 



5 = Portlandian. 

 4=Kimmeridge Clay. 

 3=Corallian. 

 2 = Oxford Clay. 

 1 = Cornbrash. 



Note. — The base of the Upper Cretaceous rocks is shown by a dark line in 

 .each section. 



Discussion. 



The Chairman (Mr. W. H. BZudleston) said that for the second 

 time that evening it had been shown to the Society that much still 

 remained to be done in British geology. The lateness of the hour 

 would preclude anything like an adequate discussion. The deter- 

 mination of the two systems of faulting, one of mid-Tertiary, and 

 the other of pre-Upper Cretaceous age, was of great importance. 

 The disturbance of which the Broadway fault was a member had 

 been held by a distinguished geologist as dating from early Quater- 

 nary times, whilst he (the speaker) admitted that he had been 

 disposed to connect it with the one through the Isle of Purbeck. 

 The subject of overthrust was most interesting, but could only be 

 criticized by those whose knowledge of the country was of recent 

 .date. 



Mr. Clement Beid also spoke, and the Author replied. 



