Revieics — A. Stra/ian — Geology of Purhech 8f Weymouth, 171 



Ft. iu. 



Lower 

 Chalk. 



( I Smooth chalk with conchoidal fracture 



Zone of t and five partings of pale -blue marl ... 



Belemmtella plena, j Smooth chalk, passing into 



\ Fine marl 



Alternations of chalk and marl in bands of 1 to 2 feet in 



thickness : an occasional line of green nodules, like 



\ those of the Chalk Rock. Ammonites varians occurs 



in the lower 22 feet, but not above ... 



Hard chalk-marl weathering into rhombs 



Very hard splintery chalk 



Sandy and glauconitic marl 



Chloritic marl; glauconitic sand with rolled casts of Jmmonites, 



^ Nautili, and ExogyrcB, the last appareutly indigenous 



4 







2 







5 







26 







6 







1 



4 







10 



4 



The westward attenuation of the Chalk is a noteworthy feature, as 

 shown in the following table (p. 164) : — 





Ballard Point. 



"Worbarrow, Lul worth, 

 and White Nothe. 



Black Down 

 and westwards. 



Upper Chalk. 

 Middle Chalk. 

 Lower Chalk. 



ft. 

 1,049 

 102 

 149 



ft. 



956 



113-90 



114-104 



ft. 

 300-500 1 



}l00-150 



The fine features presented by the Chalk in the coast sectiuns 

 are well illustrated, including the classical section of the Isle of 

 Purbeck fault on Ballard Cliff. These will be more appropriately 

 noticed under the heading " Disturbances and Faults." 



The full description of the Tertiary strata of the Hampshire 

 Basin is reserved for another memoir, but a few notes on 

 occurrences in this district are given, and these have been partly 

 furnished by Mr. C. Keid. Bearing in mind that the most 

 important industry of the northern half of the Isle of Purbeck 

 is especially connected with the Bagshot Beds, which cover nearly 

 half the area of the map, plate viii, any evidence as to their 

 physical history cannot fail to be of interest. Westwards they 

 are said to become very gravelly, and they have been found to 

 cut through the London Clay so as to rest directly on the Chalk. 

 These gravels may be distinguished from those of the Eeading 

 Beds by their containing, in addition to flint and chert, so much 

 quartz and subangular Paleeozoic rocks " as to make the fine- 

 screened material look like a Cornish beach." The history of the 

 outliers on Bincombe Down and on Blackdown, near Portisham, is 

 verj' remarkable ; and Mr. Strahan considers that the Bagshots on 

 tlie former hill have been partly affected by the Eidgvvay fault 

 " tilting up the south side of the Bincombe outlier, but leaving the 

 northern half horizontal." Formerly these outliers were regarded 

 as Eeading Beds by the Geological Survey, and as Plateau gravel by 

 Prestwich. 



The subject of Pleistocene and Eecent deposits also comes in 

 for a fair share of notice ; and here the authoi', apropos of the 



1 A variable thickness of Chalk is missina; here. 



