494 LOWER OOLITIC ROCKS OF ENGLAND : 
The Norman Castle of Rockingham is said to have been built for 
the protection of the iron-furnaces in Rockingham Forest ; but, 
as the wood became less abundant, and the method of smelting 
iron with coal was introduced, this manufacture gradually forsook 
the district* 
It was not till the year 1851, that attention was again pro- 
minently drawn to the occurrence of workable ironstone in the 
Northampton Sand. Then Samuel Blackwell, of Dudley, sent 
specimens to the " Great Exhibition." Nine years later the pro- 
duction was estimated at 95,664 tons, valued at 23,4167. : and 
since that date the production has largely increased, while towns 
like Kettering and Wellingborough have expanded under the 
influence of the trade. 
Northamptonshire Iron-ore. 
The ironstone of the Northampton Sand has been worked at intervals 
along the outcrop of the formation from near Steeple Aston, Culworth, 
and Towcester to Thrapston, Kettering, and Rockingham Forest, also 
near Stamford, at Cottesmore, AVultham-on-the-Wolds, and again at 
Coleby, Waddington, Canwick, and Greetwell near Lincoln. 
In some places the ironstone is as much as 25 or 30 feet in thickness, 
but rarely can the beds be profitably worked to a thickness of more than 
10 or 12 feet. 
The ore, which is of a yellow or brown colour, yields from 25 to 40, and 
rarely as much as 55 per cent., of metallic iron.f It is more or less oolitic 
and gritty, containing grains of quartz, &c. 
The lower layers of the rock are frequently of a greenish colour or they 
comprise green cores : these are usually regarded as unprofitable beds. 
They comprise the less weathered beds which contain not only carbonate of 
iron, but some silicate and phosphate of iron. The green colour is attri- 
buted to silicate of alumina and iron; and it may be remarked that 
analyses of the brown ore, show sometimes more phosphoric acid than 
the green ore, so that the colouring matter is not due to phosphate of iron. 
The brown iron-ore is of a cellular 
FlG. 137. character with coatings of darker 
brown ore of richer quality, so that 
Structure of the Ironstone m i n places, as observed by Mr. Maw, 
the Northampton Sand. " the entire mass of the stratum is 
(Gr. Maw.) made up of the box-like structures." 
He remarks that the hard septa that 
separate the individual cuboidal 
masses, are generally distinct one 
from another.! (See Fig. 137.) 
Prof. Judd observes that this cel- 
lular structure is in some way con- 
nected with the jointing and bedding 
of the rock ; and this view is strik- 
ingly confirmed by the fact that in 
some places, as at Easton, the direc- 
tion of one set of the sides of the 
cells is found to exactly coincide with 
that of the "master-joints" of the 
superincumbent limestones. 
Eeference has previously been made to the fossils that occur in the 
ironstone (p. 166) ; and Prof. Judd remarks that the dark brown, glazed 
* Judd, Geol. Rutland, pp. 55, 99, 110. 
t Sharp, Quart. Journ. Geol. Soc., vol. xxvi. p. 376; J. D. Kendall, Trans. N. of 
Eng. lust, of Mining Engineers, vol. xxxv. p. 135 ; and Hudleston, Proc. Geol. 
Assoc., vol. iv. p. 129, and vol. xi. p. 123. 
J Quart. Journ. Geol. Soc., vol. xxiv. p. 395. 
