GEOLOGY 



varied appearances, but the most characteristic is that of a dark brown or 

 red mineral, rich in iron, filUng cracics in, and coating blocks or roundish 

 lumps of green, grey, or brown oolitic or other lighter-coloured ferruginous 

 matter concentrically, thereby producing a peculiar cellular or box- 

 within-box-like arrangement. The red beds may however consist of 

 sand coated with a pellicle of iron peroxide, which gives a regular ruddy 

 appearance to the whole. The green ore is almost entirely an oolitic 

 carbonate of iron, to the colour of which either or both silicate and 

 phosphate of iron contribute. Low down in the series a darker bluish 

 green rock is often met with, which is rejected for furnace purposes 

 because of the phosphorus it contains. Calcareous matter is no dis- 

 advantage in the ore unless it replaces the iron too much. Highly 

 pyritous mineral occurs at a few places where the rock is deep-seated, 

 and water has not been able to circulate in it ; indeed everywhere the 

 circulation of oxygenated water appears to have been the direct cause of 

 peroxidation of the mineral. 



The origin of the iron and the form of the ore in the Northampton 

 Sand cannot here be discussed, indeed it is by no means a settled question, 

 but those who wish to pursue the subject must consult Prof. Judd's 

 remarks thereon.^ 



The distribution of fossils in the ironstone beds is most erratic ; in 

 places they are exceedingly abundant as casts, or moulds ; or when the 

 beds are more calcareous good specimens can be secured, but miles of 

 ironstone cutting may be searched in vain for such. Ammonites jurensis, 

 A. opalinus and A. murchisona^ cephalopods which are characteristic of 

 distinct zones in other parts of England, appear to occur together low 

 down in the series. 



Although 30 feet is given as the maximum thickness of the ironstone 

 beds, it is rare to find more than from 9 to 12 feet worth working. 



The Variable Beds well deserve the name for they are most irre- 

 gular in character and occurrence ; they may lose their individuality in the 

 white sands above, in the ironstone series below, or more or less in both. 

 In the forms of an inferior ironstone, a red freestone, or white oolitic 

 flaggy beds, they have been extensively used for building purposes, and 

 even dug for roofing slates (New Duston) ; they have also been burnt 

 for lime. 



Since in many places where these beds cannot be identified there is 

 no apparent thickening of the estuarines above or the ironstone below, we 

 may presume that they are absent, and where they are present, therefore, 

 a local origin for the material of which they are composed is to be 

 postulated. There is reason to believe that the purer limestones of the 

 series consist largely of coral mud and sand} certainly near Northampton, 

 in the direction of Abington, such beds partly fringe and partly cover 



' John W. Judd, 'The Geology of Rutland,' etc., Memoirs of thi Geological Survey, 

 pp. 113-138. 



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