COHESION— CONCRETIONS. 627 



The concretions in the same mass are usually nearly equal: 

 hence (3) the points at which solidification in any special case 

 begins are usually nearly equidistant. The great uniformity of 

 size in the concretions of most beds of rock shows that foreign 

 bodies do not generally determine the positions of the centres, 

 although they often act as nuclei. 



Basaltic columns are a result of concretionary structure formed 

 in cooling (p. 98), in accordance with the principles just ex- 

 plained : each column corresponds to separate concretionary 

 action. The size of the columns is determined by the distance 

 apart of the points which take the lead (these points lying in the 

 centres of the columns) ; and this is determined by the rate of 

 cooling ; and this, mainly, by the thickness of the mass to be 

 cooled : the thicker the mass, the larger the columns. The cracks 

 separating the columns from one another are due to contraction 

 on cooling. 



Iron-stone, sandstone, and clayey concretions in beds of rock, are 

 examples in which the concreting is due to a mineral solution 

 penetrating a stratum of clay or sand. A solution containing 

 silica would make siliceous concretions : so also carbonate of lime 

 in solution, or a ferruginous solution, may be the concreting agent. 

 In either case the process is as has been explained: the distances 

 between the centres, being first fixed in the concreting process, 

 determine the size of the concretions, and the equality of these 

 distances the uniformity of size. 



Spherical and flattened concretions. — A mineral solution (or any 

 liquid) naturally spreads equally in all directions through a sandy 

 or earthy stratum, and makes, therefore, spherical concretions ; but 

 in a clayey rock it spreads laterally most rapidly, and so leads to 

 flattened concretions. The vertical and horizontal diameters of 

 the concretions will be to one another as the rate of spreading in 

 the two directions. 



Hollow concretions. — Flattened rings. — In a concretionary mass, the 

 drying of the exterior by absorption around may lead to its con- 

 creting first. It then forms a shell with a wet unsolidified interior. 

 The drying of the interior, since the shell is unyielding, contracts 

 it, and consequently it becomes much cracked, as in figs. 72, 73 ; 

 or, if the interior undergoes no solidification, it may remain as 

 loose earth ; or, if it solidify at the centre by the concreting pro- 

 cess before the shell forms, or after, it may form a ball within a 

 shell, with loose earth between. 



The circumstances that would produce hollow balls among sphe- 

 roidal concretions produce rings among flattened concretions or in 



