THE CORALLIAN ROCKS OF ENGLAND. 275 



oolitic grain of the hydrous oxide ;" but even the outer skin is not 

 pure oxide ; and after treatment with acid there remains a residue 

 presenting a sphere-on -sphere structure, hollow within, and composed 

 of a uniform subcrystalline material, the greater part being silica. 



.An analysis of the mass of the ore is given in Damons account. 

 A sample of the granules has yielded the following : — 



Moisture 1-21 



Constitutional water 11-29 



Ferric oxide 73*57 



Alumina and soluble silica 4-17 



Lime 1-60 



Magnesia 1-41 



Insoluble in HC1 5-53 



Organic matter, carbonic acid, and loss. . 1*22 



Phosphoric acid trace 



100-00 

 It would lead us too far from our subject to inquire the causes of 

 the changes that have brought the rock into this state, and what 

 was its original condition, which must have been very different from 

 what it now is ; but these points must be determined by chemical 

 rather than by geological considerations. 



Such are the Corallian rocks of the Weymouth district. They 

 have been separated after the Yorkshire type by authors; but 

 though the series of changes have been remotely similar, we shall 

 seo that this affords no reason for the subdivisions being contem- 

 poraneous ; and we have therefore abstained from the use of the old 

 Yorkshire names. There is nothing specially " coralline " in the 

 series as connected with Corals; and the term " Coral Rag," which is 

 an essentially lithological name, is here especially inappropriate ; for 

 such a rock occurs only in one limited locality in the whole district. 



II. The North-Dorset District. 



A considerable and important mass of Corallian rocks is developed 

 in this district; but beyond their mapping by the Geological 

 Survey, and a paragraph by Mr. Mansel Pleydell in the ' Geological 

 Magazine,' no illustration of them has yet been given. Yet the 

 district has features which are not met with elsewhere. In general 

 the rocks strike north and south ; but they have a prolongation along 

 their southern boundary far to the west of their general range. In 

 our description we shall find it best to commence atSturminstcr, where 

 the railway-cutting affords a magnificent section, which to a large ex- 

 tent serves as a key to the whole district, the other developments 

 being best understood by their relation to the rocks here exposed. 



This cutting (fig. 2) crosses the escarpment at right angles at about 

 25 feet above the level of the river Stour, which runs over Oxford 

 Clay ; and as the beds dip at about 4° E., higher beds are met with 

 in that direction till cut off by the denudation of the country where 

 the railway leaves the cutting. 



The following beds are here exposed in descending order : — 



t 2 



