SIZE AND SHAPE OF CONCRETIONS AND CRYSTALS 355 



From what has been said the inference is easy that crystals form where 

 the collection of molecules takes place in an open space, or a space 

 occupied only by liquids, while concretions grow embedded in the 

 country rock. This is true in general, but there are some exceptions. 

 Crystals like garnet, tourmaline, etc., form apparentl}^ in rock excluding 

 the surrounding grains, while calcite frequently forms crystals including 

 grains of sand naturally lying within their limits. 



The force conceived to bring together molecules into either cr\'stals 

 or concretions is a form of molecular attraction, influenced perhaps, 

 sometimes, by electrical relations. This attraction withdraws molecules 

 from the mother liquor in the immediate vicinity of the crystal and 

 builds them into its structure. This impoverishes the solution in con- 

 tact, and consequently diffusion takes place from the saturated solution 

 farther away to enrich it. As these processes continue, quasi-currents 

 of molecules similar in nature to those of the crystal converge toward it 

 from all directions and from an indefinite distance. It is not necessary 

 to postulate motion of the liquid which acts as a vehicle, though motion 

 of such medium may sometimes accelerate or otherwise influence the 

 growth of both crystals and concretions. The impoverishing of the sur- 

 rounding country rock in the vicinity of concretions is often conspicu- 

 ously indicated by a difference of color. 



Size 



Concretions vary greatly in size, some being microscopic and others 

 attaining several yards in diameter. As in the case of crystals, the size 

 of concretions is probably inversely as the rapidity of their formation. 

 Another limiting factor may be the strength of solution from which they 

 form. 



Shape 



When concretions form in a massive deposit where the circulation is 

 equal in all directions, the usual shape is globular. This is often beauti- 

 fully exhibited in sandstone. It has been observed particularly in those 

 of the Fox hills and Laramie formations. The latter case is recorded 

 in the names of the Cannon Ball river and the Pomme Blanche hills of 

 North Dakota. Such are found also in the marls and loams of the 

 Miocene and Pleistocene. When formed in shaly formations, as noted 

 long ago, the shape is usually more or less lenticular, because accumu- 

 lation goes on more rapidly in the plane of stratification than at right 

 angles to it. If shales are very impervious the concretions may be disk- 

 like, as in figure 22 d, plate 49, and figure 3, plate 50, or even sometimes 



