506 ME, M. ODLING ON THE [Oct. I913,- 



The Stonesfield Slates vary considerably in composition, the- 

 insoluble residue being chiefly quartz. 



The following table gives the percentage composition of the- 

 types examined : — 



Residue insoluble Iron and 



in hydrochloric acid, alumina. 



Sandy bed in Stonesfield ' Slates ' 46"75 1-47 



Fine-grained ' slate ' 3-31 1-26 



Medium-grained ' slate ' 1973 496 



Coarse-grained 'slate' 1244 3'60 



Coarse-grained ' slate' enclosing pebble... 2P09 5"7o 



Pebble from the above 11-94 4"84 



Pebble from another specimen 5'96 202 



VI. Petrology. 



The microscopic examination of the Bathonian rocks has brought 

 out to a marked degree the prominent part played by the Echino- 

 dermata in the building of the Bathonian limestones ; except for a 

 few minor bands, quartz is found only in the lower beds of the- 

 Great Oolite and in the Fullonian, and bedding is also only notice- 

 able in these same rocks. 



The purer limestones may be divided into three main groups : — 



(1) In which, the matrix consists for the most part of amorphous car- 



bonate of lime. 



(2) In which the matrix is entirely crystalline. 



(3) In which the matrix is partly recrystallized from an originally 



amorphous form. 



Of those belonging to the first group, the most interesting are 

 the Cream-Cheese Bed and the Fossiliferous Cream-Cheese Bed: the- 

 latter being practically uniform in character, both chemically and 

 microscopically, from all the exposures. 



Of the seventy microscope-sections examined only four can be- 

 said to belong strictly to the second group ; this includes the shelly 

 equivalent of the Fossiliferous Cream- Cheese Bed of Gibraltar 

 Quarry, also Beds 3 & 19 of the Ardley section. 



In the lower beds of the Great Oolite and in the Fullonian, 

 angular quartz-grains are usually abundant ; this is especially true 

 of the Stonesfield Slates, where the ooliths are often formed round 

 grains of quartz. 



In the coarse type of the Stonesfield Slates numerous rolled 

 pebbles occur, as also irregular concretionary masses ; in section, 

 these appear to consist of a rock somewhat similar to the fine- 

 grained type. The irregular shape of some of these suggests 

 that they were formed by contemporaneous rolling of the mud in 

 the watercourses, and that they are not fragments of older beds. 



Quartz is abundant, and, in addition to the small shell-fragments, 

 numerous small ooliths occur; fragments of echinoderms are by no 

 means uncommon, and in one section a small portion of a fish- 

 tooth is seen. 



