306 Messrs. Bucrland's and Conybeare's Observations on the 



crowned with a small cap of lias resting- on red marl. It affords beautiful spe- 

 cimens of that irregularly-constructed and knotted form of lias^ which is 

 common in cabinets under the name of landscape-marble. 



Several immediate contacts of lias with mountain limestone may be noticed 

 in the interior of the basin. Thus at Winford^ a long tongue of coarse oolitic 

 lias forms an upfilling in a depression of the limestone of Broadfield-down ; 

 and a similar contact may be seen on the inner edge of the calcareous frontier 

 to the south of Westbury and of Almonsbury. 



3. Oolite. 



It is not our intention to enter into any detail respecting this formation in 

 general, which forms rather the boundary than a constituent part of our district. 

 Its inferior portions will alone require description. 



These consist of a coarse oolitic limestone, generally termed the inferior 

 oolite or bastard freestone, resting on a thick bed of brown ferruginous 

 sand, which often contains irregular beds of calcareo-siliceous concretions, 

 known in Gloucestershire by the name of sand-bats. This sand in the 

 lower beds, as it approaches the lias, passes into a greenish-blue marl, and the 

 concretions (which are often very siliceous) are of the same colour; and 

 hence the thin stony and concretional beds in the upper part of the blue lias- 

 marl are often very similar in character to those of the sand in question. They 

 gradually, however, as the series descends, assume the grain and texture of 

 lias. 



The preceding is a general description of the lower beds of the oolitic for- 

 mation, as they exist in the neighbourhood of Bath and throughout the Cots- 

 wold hills ; and since beds of nearly the same character may be traced south- 

 wards to a distance of more than 50 miles, it is presumable that this constitu- 

 tion very generally prevails. 



In some points however of our district the sandy beds seem to have given 

 place to beds entirely oolitic. This is especially the case in the lowest beds of 

 this formation, where they rest upon lias to the south of Midsummer Norton, 

 and throughout the plain formed by the same beds near Mells, and thence to 

 the south of the Mendips as far as Shepton Mallet, where they repose imme- 

 diately on mountain limestone and the coal-measures. 



These beds consist of a coarsely-crystalline and loosely -compacted limestone, 

 varying in colour from a straw -yellow to a dull-red, and interspersed with small 

 balls of ochreous rusty powder, produced apparently by the decomposition of 

 ferruginous oolitic grains. In the absence of these grains, the rock becomes a 

 durable freestone, which, as quarried at Dundry, has been used extensively for 

 building in Bristol ; that of Doulting-, near Shepton Mallet, has been employed 



