396 PROCEEDINGS OF THE GEOLOGICAI. SOCIETY. [April 22, 



and complete in itseK, so that the hard septa separating the cubes are 

 double, and may generally be parted into distinct layers (fig. 62). 

 The following determination of 



the iron was made by Dr. Yoelc- I'ig- Q2.— Detail of Structure of 

 j£er. ih^ Oolitic Iron-ore formation^ 



Analysis No. 78, of hard fer- Northamptonshire. 

 ruginous cakes and layers, North- 

 amptonshire iron-ore deposit, near 

 Blis worth : — 



Protoxide of iron 1-352 



Sesquioxide of iron 76*588 



Phosphoric acid 0-020 



Carbonic acid 0-014 



Silica, alumina, lime, water, "j 



&c., not separately deter- V 22-076 



mined 1 



These bands of accumulation appear to have originated in the same 

 way as the dark lines represented in fig. 54, p. 390, each having ad- 

 vanced from its centre until arrested by contact with its neighbour, 

 mutual pressure against each other having determined the angular 

 form of the ferruginous shells. 



In addition to the yeUow earthy ground forming the mass of the 

 deposit, and occupying most of the cavities, two other conditions of 

 iron are found. At about the middle of the bed occurs a line of grey 

 nodules of compact carbonate of iron, each of which is environed by 

 a cellular crust of the hydrous sesquioxide ; their composition is as 

 follows. 



Analysis No. 76, by Mr. D. Porbes, of nodules of compact carbo- 

 nate of iron (upper part of fig 37, Plate XY.). Iron-ore formation, 

 Inferior Oohte, near Blisworth : — 



Specific gravity 3*58. 



Protoxide of iron 49'58 = 79*9 carbonate of iron. 



Sesquioxide of iron 5-67 



Bisulphide of iron 0'96 = iron 0*45, sulphur 0-61 . 



Protoxide of manganese 046 



Alvmiina 1-56 



Lime 3*24= 5-8 carbonate of lime. 



Magnesia 0-46 = 1 -0 carbonate of magnesi;!. 



Carbonic acid 34-64 



Phosphoric acid 0-44 



Silica 2-16 



Organic matter Trace. 



Water of combination 1'56 



100-43 



StiU. lower down, near the base of the deposit, are some green 

 patches, likewise environed by a sinuous line of the brown ferruginous 

 cake. Except in colour, they resemble in aspect and texture the general 



