Hydrate of Iron. 



189 



inch to three inches. The sides are generally unequal in size, and 

 various as to their number. The axis of the column is always par- 

 allel, I believe to the shortest diameter of the nodule. The space be- 

 tween the columns is generally only sufficient to allow of the intro- » 

 duction of the blade of a penknife. In some instances, where a 

 shrinkage has taken place, no regular forms are produced ; but the 

 seams run in all directions. I have not observed any case where 

 the seams reach to the surface of the nodule. The outer portion, 

 from half an inch to an inch thick, is compact gray iron stone. 



An examination of these nodules, leads one at first almost irresis- 

 tibly to the conclusion, that they must have been once in a melted 

 state, and suddenly cooled at the surface : and the glazed iron-black 

 appearance of their internal surface, tends to confirm this opinion. 

 And yet, I doubt whether it is necessary to evoke the god of fire for 

 a solution of this phenomenon. For the mere descication of nodules, 

 containing so much argillaceous matter, will, it seems to me, ade- 

 quately explain the appearances. Of course the external part would 

 first part with its moisture and become solid : and as the water grad- 

 ually escaped subsequently from the interior, the shrinking would pro- 

 duce fissures within ; since the already compacted crust would not 

 permit the compression of the whole mass. And as to the shape of 

 the 'pieces, resulting from the shrinkage, it seems to me that if the 

 nodule be spherical, the fissures will coincide essentially with planes 

 passing through the center of the sphere. (See No. 126.) But if 

 the shape be that of a flattened ellipsoid, the greatest shrinkage must 

 take place in the direction of the plane, which coincides with the two 

 longest diameters of the ellipsoid : and, therefore, most of the fissures 

 will be made perpendicular to that plane, so as to produce columnar 

 masses ; although there will be a proportionable shrinkage in a di- 

 rection perpendicular to the plane above mentioned, which will pro- 

 duce at least one termination to the columns : and all the specimens 1 

 which I have examined, confirm this rationale of the appearances 

 (See Nos. 124, 125.) as does also the desiccation of clay on the sur- 

 face of the earth, producing columnar masses, which stand perpen- 

 dicular to the horizon. I apprehend it will be found, that the true 

 columnar structure exists only in those nodules which are flattened. 



The existence, sometimes, of lignite in the center of these nodules, 

 is another evidence that they never could have been completely fused, 

 since the pieces of wood that were enveloped, must have been entirely 

 destroyed. I am aware that coal is sometimes found in trap rocks, or 



