426 STRATIGRAPHICAL GEOLOGY 



but the two estuarine groups are sharply divided from one another, 

 the base of the upper group being a layer of ironstone nodules 

 which rests on an eroded surface of the lower sands. The fossils 

 are chiefly marine species, such as Ostrea Sowerbyi, Modiola imbricata, 

 Perna rugosa, and Pteroperna plana, but Cyrena and plant remains 

 sometimes occur. 



The Great Oolite consists of hard limestones alternating with 

 marly clays. These beds contain Gervillia Waltoni, Astarte angulata, 

 Terebratula maxillata, Rhynchonella concinna, and many other 

 fossils, and near Fairford is a white limestone which has yielded 

 many corals, such as Isastrea explanatula, I. limitata, and Tham- 

 nastrea Lyelli. 



The Forest Marble derives its name from Wychwood Forest 

 near Burford in Oxfordshire, where it has been largely quarried. 

 It is continued into Buckinghamshire, but near Buckingham the 

 limestones thin out, leaving only the clays to represent this 

 member of the series ; these form a band which is known as the 

 Great Oolite Clay, but it is only from 10 to 20 feet thick. 



In consequence of the changes above mentioned the stage 

 presents quite a different facies in Rutland and Lincoln, where it 

 is less than 100 feet thick and consists of four members which 

 are persistent through both these countries. These are : 



Feet. 



4. The Cornbrash about 15 



3. The Great Oolite Clays . . . 20 



2. The Great Oolite limestone . . . ,, 20 

 1. The Upper Estuarine Beds . . . ,, 30 



The Upper Estuarine Beds consist of white, blue, green, and 

 variegated clays, with seams of lignite and occasional courses of 

 sandy stone ; the basement bed is a brown ferruginous nodular 

 sandstone, which rests unconformably upon the Lincolnshire lime- 

 stone in Eutland and Northampton, though in Lincolnshire no 

 such evidence of erosion has been observed. The fossils are some- 

 times marine, sometimes estuarine forms (Cyrena, etc.). 



The Great Oolite Limestone consists of soft, white, marly lime- 

 stones, separated by layers of marly clay. The prevalent fossils 

 are Ostrea Sowerbyi, 0. subrugulosa, Homomya gibbosa, and Clypeus 

 Mulleri. 



The Great Oolite Clays are of many colours blue, green, yellow, 

 and purple ; the only fossils are Placunopsis socialis and the oysters 

 above mentioned. 



The Cornbrash is as usual full of fossils ; Holectypus depressus, 

 Nucleolites clunicularis, Pseudomonotis echinata, and Macrocephalites 

 macrocephalus are characteristic and common. 



