135 



aggregates of spear-shaped crystals of sand, cemented by carbo- 

 nate of lime, show, that this large proportion may be exceeded, 

 while the foreign material is in a somewhat coarse state. 



In the formation of claystones, however, we are to consider 

 the presence of finely-divided matter suspended in, or so mixed 

 with water of infiltration in spring-time, or general saturation 

 from position, that it has nearly a semi-fluid state. A saturated 

 solution of bicarbonate, or more commonly crenate of lime, finds 

 its way into the soft mass, by frost crevices, or channels left by 

 roots, or even air bubbles, and at these points the concretions 

 commence, when no nuclei of similar chemical composition exist. 

 The finely-divided matter interposes an obstacle to the formation 

 of crystals of carbonate of lime, far greater than an equal amount 

 of coarser foreign matter would do ; and we observe, then, the 

 influence of that beautiful law in accordance Avith which rounded 

 forms are produced. In the laboratory similar forms daily 

 occur, where the presence of finely-divided and diffused bodies, 

 arrests the formation of crystals, and globular, or curved-surfaced 

 solids are produced ; as in the animal frame, the cell structure 

 causes the dissolved phosphate of lime to take the curvilinear 

 form pertainnig to organization. The claystones which are pro- 

 duced under the simple conditions here described, have no con- 

 centric structure ; a slight conformity to this structure being 

 observed, when a bubble of air, or a vacant space, marks the 

 point of commencing deposition. In other cases, a shell in its 

 calcareous composition offers a preferred nucleus, and as it con- 

 tributes its lime salt, a concentric arrangement may be noticed in 

 the forms resulting, especially after exposing them to heat. 

 Rounded masses once formed become centres, or nuclei of second- 

 ary occurring aggregates, one central mass being surrounded by 

 spheres attached ; but in all it is easy to read the influence of the 

 tendency of carbonate of lime to crystallize, and the opposition of 

 the finely-divided silt, causing the particles of both to assume 

 forms without straight bounding lines, as the polarizing force of 

 crystallization is ai'rested in all directions. 



It may be added that a great number of bodies present rounded 

 forms de[)endent on a modification of this law of restrained crys- 

 tallization, such as numerous iron ores, bog manganese, and even 



