396 RETROSPECT OF THE OOLITIC SYSTEM. CHAP. 



The Forest marble and Stonesfield beds give abundant examples 

 of oolitic grains scattered or forming layers in shelly beds., and 

 these when cut through display a great variety of nuclei, the grains 

 being somewhat irregular in size and shape. 



Again, we have near Oxford a variety of appearances caused 

 by the dispersion of comparatively few grains of oolite through 

 the substance of a compact calcareous rock ; and, as at Dundry, near 

 Bristol, these grains may be ferruginous. 



Thus we are led to the case of Rosedale in Yorkshire, where the 

 thick valuable and partly magnetic dark ironstone is really an oolite 

 of even texture, composed of sesquisilicate and carbonate of iron, 

 with little foreign admixture. It appears to have few shells in 

 its substance, just as the pure massive oolites of Bath and Ketton 

 yield few organic remains, while the laminated and other less 

 oolitic portions of adjacent rock are filled with them. 



When, however, we speak of Great oolite, Inferior oolite, and 

 the like, it is only in a general way that we refer to the mineral 

 constitution of those rocks. Oolite is a characteristic form of some 

 of the component strata, but the structure is not confined to them. 



The colour of oolite as seen in ordinary open quarries is usually 

 a pale yellow, sometimes a little embrowned or reddened by car- 

 bonate of iron. But the same stone obtained from a considerable 

 depth in the earth, and especially under a covering of clay, is 

 usually blue. The bleaching of the open-air stone is often only 

 external,- the central parts of a massive block retaining still the 

 original blue tint. The explanation is found in the action of water, 

 which by bathing the external parts of the stone alters the con- 

 dition of the iron. This has usually been regarded as originally 

 protoxide, but Mr. Church has found the blue centres of Forest 

 marble near Cirencester to contain bisulphide of iron. In some 

 cases sesquioxide of iron occurs in the blue parts. 



The formation of the oolitic grains seems to have followed on 

 the accumulation of calcareous mud; in some cases the whole of 

 this mud has become oolitic, in others segregations of distant 

 sphericles took place. In some examples carbonate of iron has 

 the same structure and the same variations, as if what had been 

 at first a carbonate of lime had been transformed to ironstone 

 by the substitution of iron oxide for lime, as Sorby has observed. 



The iron-bands are of various aspect. Where fully developed, 



