Dr Colquhoun on the Argillaceous Ore qflrwi. 69 



cement, in the same manner as the columns which compose a 

 mass of starch. 



4. Oolitic, Lenticular, or Granular Ironstone. The cha- 

 racteristic of this variety, (which is found chiefly imbedded in 

 schist,) consists in its being formed of an aggregation of round- 

 ed particles of the mineral, generally held together by the in- 

 terposition of some extraneous cement. Several remarkable 

 specimens of this variety have been described by the French 

 chemists, one or two of which deserve to be particularly noti- 

 ced. In the Annales des Mines, vol. iv. p. 355 and 633, M. 

 Berthier has described two sorts of granular ironstone. The 

 first was found at Anzin (departement du Nord,) associated 

 with coal, and was composed of globular grains of the size of 

 small peas, agglutinated together by a bituminous schist. The 

 second was obtained from the vicinity of the village of Pou- 

 rain, in the department of Yonne, and was remarkable for its 

 occurrence in a tertiary formation where it seemed to be con- 

 temporary with potter's clay. This sort was found in round- 

 ish masses of various sizes, some of them very large, irregu- 

 larly distributed through a ferruginous sandy clay, situated in 

 the neighbourhood of a bed of ochre. The globules were 

 weakly cemented together by a thin incrustation of grey co- 

 loured clay, which quickly softened when immersed in water. 

 Another remarkable specimen, in which the grains were found 

 to be strongly bound together by bitumen, is described by 

 MM. Combes and Lorieux. * It was taken from a pretty 

 thick bed, in the immediate vicinity of a stratum of coal at 

 Lasalle (Aveyron.) The grains were of a greyish colour, 

 which strongly contrasted with the dark hue of the cement. 

 The composition of the ore was rather remarkable, and as it 

 differs materially from any of the analyses of the British spe- 

 cimens which have been given in an earlier part of this treatise, 

 we shall take this opportunity of mentioning it here. The 

 principal constituent was carbonate of protoxide of iron (in 

 the proportion of 62 per cent.) mixed with small quantities of 

 the carbonates of lime and magnesia, and with not less than 

 17 per cent, of bitumen and water, and 11 1 per cent, of sul- 



• Annates des Mines, viii. 631. 



