394 RETROSPECT OF THE OOLITIC SYSTEM. CHAP. 



the surface for the drift and settlement of the fine sand of Midford 

 and Frocester : on this sandbank flourish colonies of coral and 

 shells, and constitute the basis of the Inferior oolite. Depression 

 follows ; the deposit again becomes argillaceous ( fuller' s-earth ; ' 

 shallow water succeeds, and the Stonesfield banks of sand and shells 

 appear, followed by the Great oolite rock. Less distinctly the 

 same things occur and recur ; and the cornbrash ends this series. 



Next we have a long depression marked by 600 feet of Oxford 

 clay, followed by the fine sandbank of calcareous grit, on which 

 corals and oysters and many forms of life grew in profusion. 



Again the same things are repeated for the Kimmeridge clay, 

 Portland sands, and Portland oolite. 



It deserves remark that the three orders of deposits, clays, sands, 

 limestones, are so much alike in the several groups as to be in 

 fact hardly distinguishable by hand specimens; they seem all to 

 have been derived from similar sources from neighbouring shores 

 and lands, with no importations from afar. The oolitic limestones, 

 however, offer some peculiarities worthy of consideration. 



Oolite, the title of a large series of the calcareous rocks (included 

 in this section), is properly applied to those portions which contain 

 spherical or ellipsoidal grains, of nearly equal magnitude, cohering 

 together or cemented by intervening matter. They are usually 

 small, as the roe of ordinary fishes ; whence the synonym of roe-stone 

 (German rogenstein) ; but sometimes large as peas, and then called 

 pisolite. In a general view we may include, for comparison, 

 the spherical dolomitic concretions of Sunderland. Each of the 

 spherical grains is formed of more or less distinct concentric sheaths 

 crossed by radiating fibres. The corresponding parts in the dolomite 

 alluded to are of decidedly crystalline structure. It is possible 

 that this may be true of some oolites, but in general the small 

 masses appear to have been gathered by attraction out of calcareous 

 mud round nuclei of previously solidified matter minute fragments 

 of coral, echinida, crinoids, foraminifera, and various shells or, 

 in some cases, round grains of sand. Finally, the oolitic structure 

 is exhibited in a large, rude, and irregular form in the pea-grit, 

 of the Inferior Bath oolite, and the pisolite of the Oxford series. 



We may distinguish several conditions of the oolitic aggregates. 



The oolites of Ketton and Painswick are remarkable for con- 

 taining in large proportion beds of purely oolitic texture, the grains 



