1868.] MAW VARIEGATED STRATA. 397 



earthy ground of the deposit, being made up of incoherent Oolitic 

 grains. The following is their composition. 



Analysis 'No. 77, by Mr. David Forbes, of green patches at base 

 of Northamptonshire iron-ore deposit (fig. 37, Plate XV.). 



Protoxide of iron 40-93 



Sesquioxide of iron 6"14 



Protoxide of manganese 0*16 



Alumina 8-08 



Lime 3-47 



Magnesia 2'21 



Potash 019 



Soda 0-27 



Sulphur trace 



Carbonic acid 22*32 



Phosphoric acid 1-99 



Silica 904 



Water 492 



99-72* 



The specific gravity at 60° Fahr. was found to be 3-401 ; and an 

 examination by the microscope showed it to consist almost entirely 

 of two mineral constituents — the one crystalline and colourless, being 

 chiefly carbonate of iron, and the other of a green colour, probably 

 silicate of alumina and iron. Whether the green colour is due to 

 it or to the presence of phosphate of iron is not decided, but it ap- 

 pears probable that a green silicate does exist in the mineral. 



It may be roughly estimated to consist of 



80 per cent, of carbonate of iron, 

 7 per cent, of carbonates of lime and magnesia, 

 Hi per cent, of silicates of iron and alumina with phosphoric acid, 

 and 1^ per cent, of water. 



It will be seen, therefore, that the composition of the grey nodules, 

 and likewise of the friable green patches, is essentially different from 

 that of the mass of the deposit, and, furthermore, that their isolated 

 disposition excludes the possibility of a separate mechanical origin. 

 The question then arises, whether any of the states of combination 

 in which the iron now occurs in the bed, was its primordial con- 

 dition, and which of the other conditions have been subsequently 

 iuduced. The nodules of subcrystalline carbonate of iron forming 

 the upper grey course are clearly of secondary origin, and appear to 

 have been segregated out of the general mass ; and the lower green 

 patches do not differ much from them in composition, though retain- 

 ing the original oolitic structure of the rock. Mr. Sorby (Proceed- 

 ings of the Geological and Polytechnic Society of the West Eiding of 

 Yorkshire for 1856-57, p. 457) has shown that, in the Cleveland 

 iron ore of the Lias Marlstone, carbonate of iron has become sub- 

 stituted for carbonate of lime in fossil shells and oolitic grains ; and 

 it appears probable that these similar masses in the Northampton- 



* The composition closely resembles that of the Cleveland iron ore analyzed 

 by Mr. Dick, and given at page 57 of the 12th volume of the Quarterly Journal 

 of the Society. 



