128 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 1 7, 



Rhynchonella inconstans, M. C. Panopaea depressa, id. t. 23, f. 9. 



Ostraea deltoidea, M. C, occurring in Serpula variabilis, Fitton, Mem. t. 23, 



masses. f. 7. 



Exogyra virgula, Fitton, Mem. t. 23, triseriata, id. t. 23, f. 8. 



f. 10. 



13. Portland Oolite. 



Swindon Hill, in the line of section, PL VII., may be described as 

 an isolated mass of Portland Oolite, capped with the lower beds of the 

 Wealden (Purbeck) series. This Oolite by no means occupies a con- 

 tinuous line along the base of the Wiltshire Hills, as the section 

 through Stratton St. Margaretts shows it to be entirely absent along 

 that line, where the chalk-group rests immediately upon the Kim- 

 meridge Clay. As shown by Dr. Fitton, the Portland Oolite is 

 exceedingly patchy ; and the map illustrating his memoir " On the 

 strata below the chalk*" well shows the isolated masses of this rock 

 in Wiltshire. 



The Portland rock seems to have been the first upraised land from 

 the oolite-sea, and, as such, formed a theatre for the dwelling of land 

 and fresh-water existences which mark the Wealden deposits : hence, 

 the Wealden would be forming on the isolated peats of Portland 

 at the same time that Greensand and Gault were being deposited 

 in the surrounding seas ; thus affording an example in proof that 

 stratigraphical schemes which represent such beds as the Wealden, 

 when on the top of the Portland Oolite, Coral-rag, and Bradford Clay 

 (to confine our remarks to the Oolites), as of different age from the 

 beds surrounding them, is not always correct. This view is founded 

 upon the fact, that Portland Ooolite, with only a capping of Wealden, 

 occurs in North Wilts in occasional knolls or sub-hills ; but, where it 

 is absent, the continuity of the succeeding beds is not at all altered, 

 nor their conform ability interfered with. 



The Portland Oolite has been much denuded since its deposition, 

 and upon its washed surface the dirt-bed, which dips into its hollows 

 and fissures, has left its remains, as seen in the section at Old Swin- 

 don Hill. 



The following sections will give a tolerably correct notion of the 

 composition of the Portland rock at Swindon, the furthest limit 

 of our observations in this memoir : — 



A Section of the Upper Beds at the large Stone-quarry. 



ft. in. ft. in. 



6. Clay 8 -\ ^ . r 2. Brownish Marl, mixed 



5. White Marl-stone 2 6 1 J -| I with pebbles and gra- 



4. Band of Clay 1 f £ £ "S vel, containing vegetable 



3. White Marl- stone 3 6 J Ph w ^ matter. ( = Dirt-bed.) 3 



1. Sandy Oolites with thick beds of sand: the sand in oblique ft. in. 

 laminae : to the bottom of the quarry 20 



The next section illustrates the nature of a sand- and stone-quarry 

 at the base of Swindon old town : — 



2. Portland building-stone, about 10 feet. 



1. Portland Sands, with thin partings of stone, about 25 „ 



* Trans. Geol. Soc. 2nd ser. vol. iv. p. 103, &c. 



