68 Dr Colquhoun on the Argillaceous Ore of Iron. 



appearance assumes the resemblance of bituminous schist, and, 

 occasionally, even of certain coals. 



2. Nodular or Reniform Ironstone. The characteristic of 

 this variety is its occurrence in detached, subglobular, or irre- 

 gularly shaped masses. A considerable variation is found to 

 prevail among the different specimens in this class. Thus the 

 structure, which is generally uniform and compact, is some- 

 times changed to concentric lamellae, especially when the ex- 

 ternal surface has undergone decomposition. Sometimes, also, 

 the nodules, instead of being solid throughout, contain a cavity 

 in the centre. This cavity is occasionally empty, at other 

 times it contains some extraneous body. In most cases, this 

 enclosed matter consists of crystallized carbonate of lime. It 

 is sometimes abundant enough to fill the entire hollow, though 

 generally it merely occupies it partially, in the form of a regu- 

 lar geode, shooting out at the same time in thin veins towards 

 the exterior surface of the nodule. A specimen of this de- 

 scription formerly received the name of septarium, or ludus 

 Helmontii. In other instances, which are of much rarer oc- 

 currence, the cavity includes a quantity of the ironstone itself 

 either in the state of a powder or in the form of a detached 

 and solid ball. When the latter case occurs, and the entire 

 nodule is shaken in the hand, a rattling sound is heard to pro- 

 ceed from the interior, and it was on listening to this property 

 of a somewhat rare variety of ironstone, that the ancients con- 

 sidered it to be a quality sufficiently unusual and unaccount- 

 able, to justify the idea that the stone itself possessed some 

 excelling and peculiar virtue. They accordingly held it to be 

 an amulet of great value, and esteemed it a sovereign preser- 

 vative, and remedy against diseases. It received the name of 

 cetites or eaglestone, from a belief that the eagles transported 

 it to their nests. 



S. Columnar or Concretkniary Ironstone. The occurrence 

 of this variety of the ore is rather uncommon. It consists of 

 columns which are aggregated together, and lie in a direction 

 parallel to each other. These concretions are sometimes con- 

 nected by a thin crust of carbonate of lime, but at other times 

 they appear to be aggregated together without any extraneous 



