60 THE RIDGWAY FAULT. 







Dorchester " is a diagram of what he there saw when the railway 

 cutting was made, and he describes it as the best, and indeed the 

 only record of what was then to be seen. This work, I believe, is 

 now very rare, but the note is quoted in Damon's "Geology of 

 Weymouth." As a result of the fault a large exposure of the 

 Purbeck beds takes place at Ridgeway, and a full description of the 

 sections was published by the same author in the Transactions of 

 the Cambridge Philosophical Society for 1855. From the Ridg- 

 way sections of the Purlieck strata and at Swanage, Mr. Fisher 

 obtained a remarkable collection of insects, which are now 

 deposited in the "Woodwardian Museum at Cambridge. From 

 France, however, curiously enough, originates one of the most 

 complete accounts of the Dorsetshire chalk in a prize essay by M. 

 Barrois published in a memoir of the Societe du !N"ord, in which 

 the character of the various beds, and their thickness in the 

 different sections are laid down, and various zones are traced in the 

 white chalk, determined by their characteristic fossils. The 

 occurrence of these is established throughout the entire area with 

 great care, and, I believe, with equal accuracy, and the essay is a 

 testimony to the zeal of a foreigner working under difficulties in a 

 strange country. 



I have now given a general sketch of the position of the subject, 

 and have endeavoured to describe the importance which, at so early 

 a date in the history of geological science, was attached to the 

 district in front of us by the investigators of the period. I will 

 now proceed to the more special aspect of the district namely, the 

 character of the Ridgway fault itself. 



Webster, in his paper before mentioned, has already shewn that 

 the white chalk, as it is traced from the Isle of Wight to White 

 Xose Cliff, is often found to be quite vertical, but frequently dips 

 at a very high angle northwards. From White Xose to Bridport 

 the dip varies from 10 to 40 northwards ; whilst its mean 

 elevation along this ridge is estimated at about 500 feet. Damon 

 places its thickness at Blackdown, near Weymouth, at 800 feet. 

 The nnneralogical character of the chalk in this district and the 



