﻿216 DE. H. C. SOEBY OF THE APPLICATIOl^ OP [May I908, 



plane of stratification, the unequi-axed particles were deposited as 

 they fell horizontally. Subsequently the material settled in a 

 perpendicular line, so that the unsymmetrical structure was in- 

 creased. If these suppositions are allowable, all the facts seem 

 explained. If the inclined plane of symmetry was produced long 

 after deposition, it would indicate that the rock as a whole was 

 somewhat compressed in a line perpendicular to this plane, so that 

 it is analogous to one having a very imperfect slaty cleavage ; but, 

 considering all the circumstances, this seems extremely doubtful. 

 Possibly, however, both explanations may be true in particular 

 cases. 



Concretions formed in situ in Stratified Rocks. 



Their actual size is of secondary importance, and may vary 

 greatly. The most important consideration is the relative length 

 of their axes, which may be called a, b, and c, a being the shortest 

 and c the longest. This may depend, to a considerable extent, 

 on the length of the axes of the original nucleus, which we may 

 call oj, y, and z. Eound this the material of the concretion has 

 been collected ; and the thickness in different directions depends, to 

 a great extent, on the structure of the rock. When this is the same 

 in all directions in a particular plane, which we may call the plane 

 of symmetry, the thickness of the deposit may be called ^'; but 

 perpendicular to this it may differ, and may be called T', the 

 water penetrating equally well in all directions along the plane of 

 symmetry, though with greater difficulty perpendicular to it, pro- 

 bably because the flat particles of the rock lie mainly in the plane of 

 symmetry. This is assuming that, when the nucleus is elongated, 

 its thickness is constant. If this varies much, there may be a 

 corresponding variation in the thickness of the deposit ; in extreme 

 cases, therefore, the concretion may taper off" to a sharp point, or 

 show other abnormalities. We may, then, generalize the axes as 

 follows : — a?-{-2T', y-\-2T, z-\-2T. If, as in some cases, x,y, and z 

 are small or nearly equal, and T and T' are also equal, the con- 

 cretion is a sphere. In many cases, however, though £c, y, and z 

 are small or nearly equal, T' is less than T, owing to the structure 

 of the rock, and the result is that two axes are equal and the -third 

 less. In other cases x, y, and z, and perhaps also T and T\ are 

 unequal, and consequently all the axes differ. In a few cases, two 

 axes are equal and the other much longer, owing to an elongated 

 nucleus. Of course, the effect of the nucleus becomes relatively 

 less as the concretion increases in size. In some cases, owing to 

 variations in the nucleus, examples of several of the above-named 

 groups are found in the same rock, but sometimes all belong to one 

 of them. 



It seems probable that, in most cases, the concretions were formed 

 at an eaily period in the history of the rock, while the nucleus was 

 still chemically active. 



The relative length of the axis perpendicular to the plane of 

 symmetry seems to have depended chiefly on the nature of the 



