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 océur 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 tessare for their pavements, as noticed by Messrs. 
Buckman and Newmarch, in their new work on Cormium.' 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 bemg 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 Zerebratula mazillata, being almost the only fossil. This band is 115 feet above the 
1 <Tllustrations 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. 
