318 DR. A. M. DAVIES AND ME. J. PRINGLE ON [June I913, 



(v) Microscopic Structure of the Great Oolite Series. 

 [A. M. D.] 



Limestone at 98 feet. — This, the topmost bed of the Forest 

 Marble, is somewhat oolitic, the grains having large nuclei with 

 a rather thin coating ; fragments of shells, often waterworn, abound, 

 including echinodermal, bryozoan, brachiopod, and gastropod 

 fragments ; and there are also seen under the microscope what 

 appear to be waterworn fragments (up to 1 mm. by 0*3 mm.) of 

 a limestone of the same structure as the crystalline matrix in 

 which they are embedded. 



Limestone at 119 feet. — This shows an extremely fine- 

 grained calcite-matrix, in which are set («) a very few quartz- 

 grains, measuring about '05 mm. in diameter ; (b) more frequent 

 grains of clear calcite, of about the same size ; (c) abundant 

 waterworn calcareous fragments with a dusky appearance, also of 

 the same size or a little larger (up to 0:1 mm.), including some 

 foraminifera and echinoderm-fragments. 



A limestone at 123 feet is very similar. 



The top bed of the Great Oolite (at 136 feet) has a very 

 finely-crystalline calcite-matrix, with foraminifera, echinoderm- 

 fragments, shell-fragments, and rolled minute grains of dusky 

 calcite. There are a few tubular rod-like bodies, which may be 

 sponge-spicules. These, however, are so scarce that they would 

 have been overlooked if they had not been specially searched for, 

 on account of their abundance in the ' Cream-Cheese ' bed of the 

 Bicester railway-cutting, with which the bed now described is 

 correlated. The two beds are of generally similar structure, but 

 the fragments embedded in the matrix are decidedly smaller in 

 the ' Cream-Cheese ' bed. 



Great Oolite limestones at 143, 144, 146, 155, 191, and 

 195 feet. — While varying much in coarseness of grain and pro- 

 portion of matrix to fragments, these show certain features in 

 common. They are composed largely of organic fragments — of 

 echinoderms, brachiopods, and lamellibranchs — with occasional 

 oolite-grains and many rolled fragments of limestone, conspicuous 

 by their duskiness, varying from 02 mm. up to 1 mm. in diameter. 

 In several cases, fragments of oolitic limestone occur, a fact which 

 suggests that many of the isolated oolite-grains may be derived. 

 In some beds, subangular quartz-grains are abundant ( = about Ol 

 mm. in diameter). The bottom bed of the Great Oolite (at 195 

 feet) shows scattered oolite-grains and fragments of oolitic lime- 

 stone, all with the dusky margin which appears to denote 

 derivation (PI. XXXIII, fig. 1). 



Thus, throughout the whole of the Forest Marble and Great 

 Oolite we find constant evidence that, while the limestones were 

 being deposited, other limestones of very similar character and 



