1852.] 



WESTON ON PORTLAND. 



117 



AVe must now refer to the contemporaneous Purbeck beds in tbe 

 Isle of Portland. 



Former sections * have shown that the Isle of Portland dips away to 

 the S., so that the high escarpment on the N. diminishes to a mural 

 cliif, very little elevated above the sea-level, at the Bill. This ex- 

 tremity I examined for the express purpose of determining whether 

 my suspicions of the extension of the Purbeck so far South, which 

 my hurried visit in 1847 had led me to entertain, were correct or 

 fallacious ; and I now found on its western side an unequivocal section 

 of the junction of the marine and freshwater strata, as in the accom- 

 panying sketch. 



Fig. 10. — Section of part of the Cliff at the Bill of Portland, 



Shingle-bed .. 

 r Purbeck 



Second Dirt-bed 

 Purbeck 

 First Dirt-bed 



Purbeck 

 {Valvata and Cyrena.) 



Portland 

 (Trigonice, Pectines, ^c.) 



7. Marine. 



6, Alternations of terrestrial 

 and freshwater ? 



5. Terrestrial. 



4. Freshwater ? 



3. Terrestrial. 



2. Freshwater. 



1. Marine. 



No. 1 is the Portland formation. It is quarried down to nearly 

 high^water-mark, and abounds, from the lowest portions to within a 

 short distance from its junction with No. 2, with remains of marine 

 lamellibranchiate molluscs. The beds which are immediately below 

 the Purbeck in the northern part of the island, are, on the other hand, 

 equally marked by a profusion of gasteropodous molluscs. 



No. 2. This stratum is not very decidedly separated from No. 1, 

 and its equivalent in the quarry containing such numerous casts and 

 impressions of gasteropods is Hthologically a bond fide portion of the 

 subjacent and unquestionable marine deposit. The strata 1 and 2 

 appear in situ to have been continuous and of common origin. Prof. 

 E. Forbes has, however, in his valuable paper ''On the succession of 

 strata in the Dorsetshire Purbecksf," clearly proved that there is no- 



* Trans. Geol. Soc. 2nd Series, vol. iv. pi. 2. 



t Rep. Brit. Assoc. 1850, Transact. Sect. p. 79. Edinb. New Phil. Journ. vo-l.xlix. 

 1850, p. 311. 



