GIRVANELLA IN OOLITIC EOCKS. 279 



* 



the nucleus is perfectly concentric. Viewed through a microscope, 

 spherules of this type would be regarded, at all events at first 

 sight, as of concretionary origin, and I am not prepared to say that 

 such may not be the true interpretation. When photographed, 

 however, what under the microscope are very obscure dark spots 

 appear like the extremities of tubes which have been cut across. 

 These objects occur throughout the concentric layers at short inter- 

 vals ; and it is impossible to resist the idea that the ax)parent 

 concentric layers may really be layers of tubuli which were filled 

 with dark material, and the walls of which have become crystalline. 



Portland Oolite. 



The beds of the Portland Oolite vary considerably in microscopical 

 structure. I therefore found it necessary to take a section of a 

 quarry and examine each bed separately. 



Section of the Portland Oolite. 



Imosthay Quarry, Portland. 



Beds in descending order. 



Full of the casts of Cerithium 

 portlandiciim, Trigonla gibbosa. 

 Chert at the top contains mi- 

 nute shells. 



Fine-grained limestone. Termi- 

 nates in a layer of chert. 



Inferior stone on account of lia- 

 bility to split. 



Chert nodules most numerous at 

 the bottom. 



Mass of casts of shells : Trigonia 

 incur va{2), Aminonites giganteus, 

 Cerithiiiin loortlandicum. 



CcTHmencing at the " Base Bed," we find the rock made up 

 entirely of dark oolitic spherules, which have been much altered by 

 crystallization, and nothing can be said as to the original structure. 



Carf Boacli Bed. — Contains some rather large cr3'stalline oolitic 

 spherules, numerous granular nodules, and organic detrital material. 

 The spaces between are filled with small crystals of calcite and a 

 quantity of dark granular structure, which indicates altered organic 

 calcareous remains. 



Whit Bed. — The last, or top bed, is a mass of small crystalline 

 organic fragments, with a few oolitic spherules. Dr. Sorby's* 

 description and figure of the Portland Stone is not typical of the 

 Portland Stone of commerce, namely the " Whit Bed,'' as it occurs 

 in the above quarry. 



* Presidential Address, p. 50, and Appendix, p. 70, pi. iv. (1879). 

 Q. J. G. S. No. 182. u 





ft. 



in. 



Roach Bed 



2 



() 



r^ /^ Portland Building-stone 



OJ 1 



6 







pq 1 



^ ■{ Portland Stone with numerous 



^ 1 cracks. 



^ l^ Portland Stone with Chert ... 



3 

 3 







() 



Curf Eoach 



3 







Base Bed 



8 







