OOLITE. 



183 



torted in various directions ; it also frequently loses its earthy tex- 

 ture, and is hard and semicrystalline, like transition limestone. 



The Rev. R. Halifax, of Standish, near Gloucester, obligingly 

 showed me part of the lias and oolite beds in the vicinity of Chelten- 

 ham, which he had particularly studied. Between the upper lias 

 clay and the oolite, there is a thick bed of reddish earth with ferru- 

 ginous nodules inclosing portions of lias ; this earth may be seen crop- 

 ping out at the foot of Leckhampton Hill. No well-marked natural 

 division exists, which can determine whether this bed should be class- 

 ed with lias, or the oolites. The fossils in lias clay and limestone 

 are nearly black, and are sometimes incrusted with pyrites. 



The most valuable mineral substances, obtained from lias in Eng- 

 land, are water-setting lime and alum shale. The property of setting 

 under water may be communicated to any kind of lime, by an admix- 

 ture with burned and pulverized ironstone. Many of the bituminous 

 and pyritical shales in the coal strata, if they could be obtained with 

 facility, would yield alum by slow combustion. When alum shale is 

 burned, and the soluble part is extracted by water, it is necessary to 

 add potass before the process of evaporation, as crystallized alum is 

 a triple salt, composed of sulphate of alumine and potass. 



Oolite. — The numerous beds of yellowish limestone alternating 

 with beds of clay, marl, sand, and sandstone, that compose the oolite 

 formation in England, are of variable thickness ; their aggregate aver- 

 age depth, from the top of the upper oolite to the lias, may be esti- 

 mated at 1200 feet. These beds may be traced, with little interrup- 

 tion, along a waving line from the Cleveland Hills in Yorkshire, into 

 Dorsetshire. In Gloucestershire, they compose a lofty range pf hills 

 on the south side of the Vale of Severn, called the Cotteswold Hills ; 

 but no strata of this formation are found in any part of England or 

 Wales north-west of the river Severn. In many parts of France, 

 the oolite strata, accompanied with lias, present all the characters of 

 the same formation in England ; but in the Jura mountains, where 

 they are fully developed, the mineral characters often differ consid- 

 erably ; and it is only from the geological position and the imbedded 

 fossils, that they can be identified with the English series. 



Oolite or Roestone receives its name from the small globules like 

 the roe of a fish, that are imbedded in many of the strata : in some 

 instances, these globules attain the size of a pea, and this variety has 

 obtained the name of Pisiform oolite. In England, nearly all the 

 beds of limestone that are oolitic, in this formation, have a yellowish 

 brown or ochrey colour, by which they may at first sight be distin- 

 guished from lias. The limestone in which the globules are imbed- 

 ded has generally an earthy texture, and is dull and incapable of re- 

 ceiving a polish : some varieties of oolite have been much used for 

 architecture. Somerset House, and many of the public buildings in 

 London, are constructed of this stone ; but it is not durable. The 

 occurrence of small oviform globules in limestone is not confined e-x- 



