Lower oolite. 



185 



oolite formation. The shells and bones in the oolite limestone, have 

 the yellowish ochrey colour of the stone in which they are imbedded ; 

 which may serve, at once, to distinguish them from the lias fossils, 

 that invariably, partake of the colour of the beds in which they oc- 

 cur. English geologists make three divisions of the oolite formation, 

 ' — the upper, the middle, and the loiver : they are separated by thick 

 beds of clay, and some variety may be observed in the fossils of each 

 division, but the general characters are nearly the same. In an ele- 

 mentary treatise, a too minute description would only perplex the 

 student, particularly as some of the beds appear to be of limited ex- 

 tent. 



The lower division of oolite, comprises; 1st, an imperfect dark 

 brown limestone, much intermixed with sand and the oxide of iron ; 

 2dly, beds of sterile clay and fuller's-earth ; and, 3dly, the great 

 oolite, sometimes called the Bath oolite, which is of considerable 

 thickness, and yields freestone for architecture : it is composed of 

 minute globules and broken shells, united by a yellowish earthy cal- 

 careous cement. With the lower division of oolites may also be class- 

 ed, 4thly, the forest marble and Stonesfield slate ; the latter is a 

 sandy calcareous stone, dividing into thin strata, accompanied with 

 shale and carbonaceous matter. The beds of forest marble are not 

 numerous, and are chiefly composed of large fragments of shells; 

 small entire turbinated shells abound in some of the strata. It de- 

 serves attention, that the univalve shells are most frequent in the thin 

 beds, and the bivalves in the thicker beds, of this stone ; 5thly, corn- 

 brash. This is the upper part of the lower division of oolites ; it 

 does not compose beds of any considerable thickness, nor does it fre- 

 quently occur in regular strata of any great extent, but generally in 

 detached masses, cemented by clay : the external part of the stone 

 is brown, but the inner part has often a grey or bluish colour. The 

 cornbrash is so thin a bed, as scarcely to be entitled to a place in the 

 division, but it is remarkable for the abundance of its fossils. The 

 above arrangement of the lower oolites was formed from their occur- 

 rence in Somersetshire and the vicinity, where they were first studied, 

 but it by no means represents the general succession of the beds in 

 other countries. In the eastern moorlands of Yorkshire, the oolitic 

 series are well displayed on the coast, and have recently been descri- 

 bed by Mr. J. Phillips. Two vast depositions of sandstone, shale, 

 and coal, occur below the cornbrash in the following order, ascending 

 from the lias :— 



Feet. 



1. Ferruginous beds above lias, thickness, - - 60 



2. Lower sandstone, shale, and coal, _ _ - 500 



3. Impure limestone, supposed to represent the Bath oolite, 6Q 



4. Upper sandstone, shale, and coal, _ - - 200 

 6. Cornbrash, ^---^-^ 5 



"825 



24 



