Dr Colquhoun on the Argillaceous Ore of Iron. 289 



3. This nodule was also found in the vicinity of Crossbas- 

 ket, imbedded in argillaceous schist. Specimens of the same 

 kind are found very plentifully distributed throughout the stra- 

 tum, and all of them are remarkable for an extremely irregular 

 shape. They are of rather a small size, varying from three to 

 six inches in diameter. It broke with a fine-grained fracture. 

 The colour of the fractured surface was dark-greyish black ; 

 but it was variegated in consequence of the presence of white 

 coloured specks and veins of carbonate of lime. It was compos- 

 ed of 



Carbonate of protoxide of iron, . . 81.22 

 Insoluble clayey matter, . . . 12.30 



Carbonates of lime and of magnesia, mois- ) _ ^ ^ 

 ture, and loss, .... J 



■ -Eq;ur 



100.00 



The following was the result of the analysis of an ore which 

 constitutes a pretty thick stratum in the vicinity of Crossbas- 

 ket, and we state it as furnishing a striking example of the ex- 

 cessive proportion in which the carbonate of lime is occasional- 

 ly intermixed with the carbonate of iron, or vice versa. Its 

 colour was dark-bluish black ; and it possessed all the external 

 and physical characters of the common argillaceous carbonate 

 of iron, excepting that its specific gravity was necessarily low. 



Carbonate of lime, .... 53.50 



Carbonate of protoxide of iron, . . 22.33 



Clay, carbonaceous matter, carbonate of) 



magnesia, moisture, and loss, . j 



24.17 



100.00 



Thus there was little more than one-fifth part of the mass 

 composed of the carbonate of protoxide of iron, while the one- 

 half consisted of the carbonate of lime. 



From the details which have been just stated respecting the 

 composition of these various specimens, we shall now endeavour 

 to deduce the general mineralogical character of the varieties 

 composing this distinct sub-species of the carbonate of protoxide 

 of iron. In doing this, however, it must be remembered that 



