1846.] brodie on the existence of purbeck strata. 53 



November 4, 1846. 



The following communications were read : — 



1. Notice on the existence of Purbeck Strata with remains of 

 Insects and other fossils, at Swindon, Wilts. By the Rev. 

 P. B. Brodie, M.A,, F.G.S. 



The indications of Purbeck deposits in this neighbourhood, although 

 the beds are neither very extensive nor of any great thickness, are 

 more clearly defined than has been generally supposed. There are 

 about thirteen feet of strata decidedly freshwater, containing Cypris, 

 Paludina, Planorbis, and remains of vegetables, chiefly Thuytes and 

 small seed-vessels. The western end of the great quarry at Swindon 

 presents the following section in descending order: — 



ft. in. 



1. Soil 2 



2. Wliite, soft rubbly stone divided by layers of clay 5 



3. Fine white laminated slaty stone. The lower part is very soft and 



more sandy, and contains branches of Thuytes and the wing-cases 



of Coleoptera 1 6 



4. Soft, white, crumbling bed with small Paludinse 1 ill 



5. Bhie clay about two feet thick, full of Cypris and Paludina, passing 



into a grey, earthy bed with Thuytes, seed-vessels, and other re- 

 mains of plants 4 



6. Hard white stone, very crystalline, with Planorbis and wood 1 6 



Total 15 



There can be no doubt therefore that these beds are the represen- 

 tatives on a small scale of a portion of the Purbeck formation. The 

 surface of the Portland strata has been greatly denuded previousljr 

 to the deposition of the overlying group, for in many cases the latter is 

 deposited in hollows and cavities where the Portland sand has suffered 

 erosion by water, on which it reposes at very irregular intervals. 



The top of the Portland oolite is composed chiefly of whitish sand 

 containing layers of Trigonia and other marine shells, and this in- 

 cludes harder nodules full of shells, chiefly Trigonia incurva, Lucina 

 Portlandica, Cytherea rugosa and Nerita angulata, often retaining 

 their shelly covering though decomposed and friable. These masses 

 too are very unevenly dispersed. 



No. 5 in the section, which in its lower part is highly carbonaceous 

 and resembles vegetable mould, is to all appearance a kind of ' dirt 

 bed,' and may be considered as the representative of one of the dirt 

 beds in the islands of Portland and Purbeck. It also contains nume- 

 rous small pebbles which are likewise found in it on the coast of 

 Dorsetshire. A few detached scales of fish and small teeth of Sau- 

 rians occasionally occur in some of the upper beds, but I could detect 

 wotrdces o^ ArchcBoniscus. There were also but few and imperfect 

 remains of insects ; nevertheless it is interesting to find them at all in 

 this detached portion of the Lower Wealden. 



I am aware that Dr. Fitton has, in his able and interesting paper 

 on the ' Strata below the Chalk,' alluded to the probable existence 



