442 STEATIGKAPHICAL GEOLOGY 



oolite " between them. The name " Corallian " was proposed by 

 d'Orbigny in 1849, and is now generally adopted as more con- 

 venient for the variable group of beds which intervene between 

 the Oxford and Kimeridge Clays. Two zones can be recognised, 

 which can also be called the Lower and Upper Corallian, thus : 



Upper f Upper rags and grits 1 r , , n . , . . 7 . ,,. 

 Coiallian\Coral Rag limestones) Zone ot f^sph^ctes phcatihs 



P . /Lower Calcareous Grit, Zone of Aspid. perarmatum. 



In South Dorset the Corallian has a thickness of about 200 

 feet, but in North Dorset it is not more than 120, and thence 

 northwards it varies from 80 to 100. The component beds are 

 well exposed in the cliffs south of Weymouth, where local names 

 have been given to the several members of the succession by Messrs. 

 Blake and Hudleston, 18 as below : 



Feet. 



/'Sandsfoot Grits . . . . . . 15 to 25 



Upper I Sandsfoot Clay 12 to 40 



Corallian I Trigonia Beds, shelly ragstones . . . 12 to 16 



VOsmington oolite, limestones, and marls . 40 to 60 



T . , ( Benclitf Grits, false-bedded sands . . 10 to 35 



C rillian" Nothe Cla y bluish sandy clays . . . 30 to 40 



[Nothe Grits, calcareous grits, and marls . 20 to 35 



The Nothe Grits and Clays contain a mixture of Oxfordian and 

 Corallian species Gryphcea dilatata, Gardioceras cordatum, Ostrea 

 yregaria, and Trigonia perlata. The Osmington and Trigonia 

 Beds represent the Coral Bag, but corals are rare ; the rags contain 

 Trigonia clavellata, Gervillia aviculoides, Nerincea Goodhalli, Am. 

 2)licatilis, and many other fossils. The Sandsfoot Clays and Grits 

 yield Astarte supracorallina, with Thracia depressa, Ostrea deltoidea, 

 Rhynchonella inconstans, and Belemnites nitidus, which range into 

 the Kimeridge Clay. 



Near Abbotsbury (see Fig. 148) the Sandsfoot Grits pass into 

 ferruginous sandstone and oolitic ironstone, the latter quarried as 

 ore. Similar ironstone occurs at Westbury in Wiltshire, in which 

 county the general succession is : 



Feet. 



( Ironstone I Upper Calcareous Grit . . 15 to 20 



Upper | Sands and clays / 2Q tQ 3Q 



Corallian I Rubbly oolite and coral beds \ c j j. 2Q t 5Q 



VCalne Ireestone | 



Lower Corallian, sands with calcareous burrstones . . 50 



The freestone thins out rapidly north of Calne, and is not 

 present at Faringdon, where the Upper Corallian is not more 



