IRON ORES. 495 
surfaces of the casts of the fossils in the weathered ironstone-rock may be 
accounted for, when we consider the tendency which there will be for 
water to accumulate in the spaces left empty by the solution and removal 
of the substance of the shells. 
He further remarks that " When the Northamptonshire iron- ore is met 
with at the bottom of very deep wells, it is, throughout its whole thick- 
ness, composed of the hard, compact, grey carbonate, and this is also the 
case when it is dug beneath beds of Boulder Clay * * * From a com- 
parison of the various analyses it appears that the grey carbonated mineral 
in the Northamptonshire iron-ore, consists of from 60 to 80 per cent, of 
carbonate of iron, with from 10 to 25 per cent, of insoluble matter, princi- 
pally sand and oolitic siliceous concretions ; besides these, and existing in 
smaller proportions, we find the carbonates of the alkaline earths and 
alkalies, water, carbonaceous matter, sulphur, and phosphorus."* 
Analyses show that the yellow ore contains less, and the brown ore 
more, iron than the grey ore, whence both were derived ; the iron in the 
weathered beds being as it were more concentrated. Thus the weathering 
or oxidizing process causes n diminution in the bulk of the rock, to the 
extent of about 12 per cent. ; and consequently we find the open joints 
and increased porosity of the rock along the outcrop. t Excepting in one 
place near Greetwell, the iron-ore is obtained in open quarry- workings by 
the removal of the ' overburden ' ; but it was formerly wrought in under- 
ground workings at Cogenhoe and Woodford."J 
Prof. Judd states that in connexion with the grey, carbonated condition 
of the rock, " there are two circumstances of great interest and importance 
which require remark : firstly, that the fossils in it retain their shelly 
substance, though the carbonate of lime is frequently replaced by carbon- 
ate of iron, and are not, as in the weathered rock, merely double casts ; 
and secondly, that there occur in the beds [pieces] of lignite, vertical plant 
remains and fragments of carbonaceous matter." 
He remarks that "these ironstones vary greatly in thickness, occa- 
sionally constituting the whole of the formation, frequently forming the 
greater part of it, not seldom being reduced to very small proportions, 
and sometimes being wholly wanting "; moreover the " ironstone when 
present always constitutes the lowest portion of the formation and lies 
immediately upon the Lias Clay." 
A careful examination of each tract of ground, accompanied in most 
cases by trial-holes and also by analyses of the rock, would be necessary 
to prove the occurrence of workable beds of ironstone in regions adjacent 
to those where the ore is now worked. 
Observations on the probable source or origin of the iron-ores will be 
left for the volume dealing with the Middle and Upper Oolites. 
The following statements concerning the structure of the rocks are by 
Prof. Judd : 
"Notwithstanding their great differences in Uthological and chemical 
characters, the different varieties of the Northamptonshire iron-ore 
namely, the gray carbonate, the light coloured earthy peroxide, and the 
hard, dark coloured laminae, the microscopical features presented by them 
all are essentially the same. In all, a slight examination is sufficient to 
show that there is a considerable amount of variation in the intimate 
structure of the rock from different localities, and of the different beds in 
the same locality ; in some cases the whole mass is seen to be made up of 
oolitic grains, varying in diameter from -^ to y^ of an inch ; in others the 
structure of the rock is seen to be throughout compact or granular ; and 
in other cases again, and these are by far the most frequent, we find a 
compact matrix, with oolitic grains disseminated through it in greater or 
less abundance. In nearly all cases there occur, scattered throughout the 
mass, rounded or sub-angular grains of quartz." 
" When fragments, not pulverized, of the Northamptonshire iron-ore 
are digested in hydrochloric acid, a white mass is left nearly equalling in 
* Geol. Rutland, pp. 116, 127, 133, 136. 
t Kendall, op. cit., pp. 147, &c. ; and Iron Ores of Great Britain, 1893. 
JSee R. Meade, Coal and Iron Industries, 1882, p. 546. 
