4 MOLLUSCA FROM THE GREAT OOLITE. 



economic purposes : their thickness is about twelve feet. A soft, shelly sandstone, called 

 oven-stone, next occurs : the shells increase in quantity downwards : about six feet will 

 represent its thickness. To this succeeds the weatherstones, consisting of several beds, the 

 aggregate thickness of which is about six feet. These lower beds are very shelly ; but, owing 

 to the greater hardness of the matrix, specimens cannot be extracted in any considerable 

 number. The blue or brown clays of the Fullers-earth support the weatherstones, without 

 any appearance of Stonesfield slate. It is also absent in several other limited shelly 

 deposits ; but, as a general rule, throughout the district, the Great Oolite, near to its base, 

 has one or more beds, which possess all the essential characters of Stonesfield slate. A little 

 higher in the series than the shelly beds, the limestones occur which cover continuously a 

 very considerable area upon both sides of the vale of Chalford, and continue upwards, with 

 various modifications of character, even to the Bradford clay. The lowest of this series is 

 a very compact cream-coloured semi-siliceous, but argillaceous limestone, four feet thick, 

 divided into two beds. It is usually destitute of organic remains ; but in some localities 

 contains casts of species of Purpuroidea, of several species of Natica ; and, also, at a single 

 locality, a dense colony of our new genus Pachyrisma, which has not hitherto been found 

 in any other stratum. This limestone extends even to the vicinity of Cirencester, and was 

 employed by the Romans to form tessarse for their pavements, as noticed by Messrs. 

 Buckman and Newmarch, in their new work on Corinium. 1 The base line of the white 

 limestone is 60 feet above the Fullers-earth at Minchinhampton, and 45 feet, four miles to 

 the east of that place, near to the railway (Sapperton tunnel) ; the measurements have 

 been obtained by well-sinkings. Above this rock occurs a series of pale brown or 

 chocolate-coloured limestones, sometimes compact, sometimes sandy, having between them 

 an occasional uncertain band of marly clay. These clays are always fossiliferous, abounding 

 in casts of bivalve shells, which have both valves generally united. The uppermost 40 feet 

 of this series, owing to the worthless character of the stone, is very imperfectly exposed, 

 our knowledge of it being chiefly derived from pits of no great size, opened for the repair 

 of the roads. The eastern extremity of the railway tunnel (Sapperton) offers an extensive 

 section of these beds, but their position does not allow of their being studied, except at a 

 distance. The white limestone is exposed about the middle of the section. One of the 

 road-side excavations, two miles east of Minchinhampton, and 90 feet above the Fullers- 

 earth, has two beds of sandy limestone which is more than usually fossiliferous, they 

 expose sections of Nerinea, Pterocera, Natica, Cylindrites, Bulla, Purpuroidea, several of 

 the Echinodermata, &c. The bivalves, which are more numerous, comprise Pholadomya, 

 Homomya, Ceromya, Lucina, and Cercomya. The shell is preserved in the condition of 

 crystalline lime, but the interior mould only can be extracted entire. At three miles and 

 a half east of Minchinhampton, a large excavation has a band of brown clay, which abounds 

 with Terebratula maxillata, being almost the only fossil. This band is 1 1 5 feet above the 



1 • Illustrations of the remains of Roman art in Cirencester, the site of Antient Corinium,' by Professor 

 Buckman, F.G.S., and W. C. Newmarch. London, 1850. 



