COHESIVE ATTRACTION. 629 



examples in which the concreting is due to a mineral solution pene- 

 trating a stratum of clay or sand. A solution containing silica would 

 make siliceous concretions : so also a calcareous or a ferruginous solu- 

 tion 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 concre- 

 tions 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 process, 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 

 clayey layers. They arise from the solidification commencing first 

 around the circumference of the concretions, and then the circle thus 

 begun acting as a nucleus about which the concreting is continued. 



The concentric coats in many concretions are due to an intermittent 

 action in the concreting process. If a drop of a weak solution of 

 sugar dry upon a slab of stone in the air, there will result concentric 

 rings. The outer edge of the circular spot dries most rapidly ; and, 

 when solidification begins along it, the liquid inside of it, for a limited 

 distance, is drawn to the concreting circle, exhausting the sugar for 

 that distance inward ; then the spot of dissolved sugar, thus made 

 smaller, concretes again at its outer edge, and forms in the end a new 

 circle ; and so it goes on until all is evaporated. A concentric ar- 

 rangement of colors and of layers is often thus produced in 'ferrugi- 

 nous concretions, the outer shell first drying and concreting, and after- 

 ward successive concentric shells, to the centre. 



3. Capillary Attraction. — The process resulting in concentric 

 coats, described in the preceding paragraph, is due partly, as is seen, 

 to capillary attraction. In the drying of the soil during dry seasons, 



