1866.] The New Ivon-fields of England. 327 
iron-stone occurs, but it is as yet unrecognized, practically at least, 
in the Coteswold Hills and in Somersetshire. 
The iron-stone is the upper member of the Marlstone, or Middle 
Lias formation, the lower consisting of sandy shales, or fine lami- 
nated sands with bands or nodules of iron-stone. Where the upper 
member ceases to be sufficiently rich in iron to be deemed an iron- 
stone, it occurs in the form of a hard calcareous grit, generally very 
full of its characteristic fossils, such as: Rhynchonella tetrahedra, 
Ri. variabilis, Terebratula punctata, Pecten xquivalvis, Nautilus 
truncatus, Ammonites margaritatus, and <A. spinatus. As it 
is overlain directly by the soft shales of the Upper Lias (or “alum 
shale”), these in the process of denudation have often been 
washed away, leaving platforms of the rock projecting from the 
flanks of the range, or forming isolated flat-topped hills rismg above 
the general surface of the country. Oxenton Hill in Gloucestershire 
is a good illustration of the latter case; and the section above 
(Fig. 1) represents the former. ; 
Fie, 2—A GLovucesTersHirE Hin. 
—SS — SSS 
SS == —— 
In the above figure the upper conical portion of the hill is composed of Upper Lias, capped by 
Oolite, the platform on which it rests being Marlstone, or Middle Lias. 
The oolitic iron-stone of Northamptonshire, and a few other 
places, belongs to the lower portion of the Great Oolite, called by 
the Geologists of the Government Survey “the Northampton 
Sands,”* and is very irregular in its distribution, although occupying 
on the whole a large tract of country. These sands rest directly 
on the Upper Lias Clay, and are overlaid by the white limestones 
which form the upper member of the Great Oolite. The iron-stone 
is found in irregular beds, sometimes reaching a thickness of twelve 
or fourteen feet, and within a mile or two thinning away altogether. 
There is a remarkable instance of this near Blisworth, for the iron- 
stone, which occurs plentifully at the northern side of the railway 
tunnel, disappears altogether at the southern end. The ore is 
a hydrated oxide of a rich rusty-brown colour, of high specific 
gravity, and silicious. When reached at some distance under- 
ground, where it has been protected from atmospheric influences, 
the colour is found to be olive-green, but these portions are generally 
rejected by the quarrymen. It is extensively worked at Blisworth, 
Gayton, Glendon, Maidford, Wellingborough, and Duston. In all, 
there are about thirteen quarries and four furnaces in blast in North- 
amptonshire, besides which, large quantities of the stone are sent by 
* See memoirs ‘On the Geology of Parts of Northamptonshire, by Mr. W. T. 
Aveline, F.G.S. 
VOL, III. Z 
