GIRVANELLA IN OOLITIC EOCKS. 27' 



The Great Oolite. 



• 1. The Stonesfield Slate. — Specimens were collected from Eyford 

 in Gloucestershire, on the eastern side of the Cotteswold Hills, where 

 the so-called " slate " has been worked since the time of the Romans. 

 Dr. 8orby describes the Stonesfield Slate thus * : — '• This is a good 

 example of a fine-grained shell-sand with small grains of quartz. 

 Most of the fragments are of Brachiopods and Oysters, which are of 

 flat shape, and thus cause the rock to split easily along the plane of 

 bedding." I find this description to apply only to the " Base Bed " 

 at Eyford (^. e. the lowest which rests on the Fuller's Earth) and not 

 to those above. The beds which give the " best slate " are made up 

 of broken valves of Ostracoda, a few Foraminifera, and a quantity of 

 undetermined material, which, whatever it may be, does not consist of 

 fragmentary remains of Brachiopod and Oyster-shells. There is a large 

 amount of insoluble residue, which includes detrital quartz, zircon, 

 tourmaline, rutile, and microcline. No oolitic structure was observed 

 io these beds, nor any sign of Girvanella. 



2. The Bath Oolite. — Of this rock it is most difiicult to prepare 

 sections for microscopic examination on account of its softness and 

 the slight cohesion of the oolitic spherules with the crystalline matrix. 

 My specimens were collected from Coombe Down, near Bath, and 

 Cross Hands, in Gloucestershire. 



The specimens from Coombe Down show a highly crystalline lime- 

 stone, and consequently a large percentage of calcite. The oolitic 

 spherules were so crystalline that in some cases the concentric 

 structure had disappeared. All the specimens show fragments of 

 shells, and in some cases Foraminifera are met with. The specimen 

 from Cross Hands showed the same crystalline condition as those 

 from near Bath, and only a few of the spherules were preserved, 

 most having been detached during the process of preparing the slide. 

 No reliable evidence could be obtained as to their origin. 



3. Forest-Marble. — The specimens were collected from Langton 

 Herring, near Weymouth, and from a boring at Swindon, Wiltshire. 

 In both cases the limestone partook of the Bath-oolite character, 

 but'it is more shelly, and oolitic spherules are not so numerous. 

 Like the former, too, the spherules are so crystalline that little can 

 be said as to their origin. 



Passing over the intermediate formations, I take next the Coralline 

 Oolite. All my specimens were collected from near Weymouth, and 

 reference has already been made to my communication to the 

 ' Geological Magazine,' in which I have shown that the pisolite 

 spherules in the Oolite near Weymouth and at Sturminster Newton 

 are not concretions but a minute form of Girvanella f. 



I find that preparations of this rock for microscopic examination 

 are seen to most advantage when not covered with Canada balsnm. 

 It is best to simply polish the surface of the thin sections, and then 



* Presidential Address, Quart. Journ, Geol. Soc. p 53, and Appendix, p. 70, 

 plate vii. fig. 1 (1879). 



t Geol. Mag. dec. iii. vol. vi. p. IDG. 



