354 J. E. TODD — CONCRETIONS AND THEIR GEOLOGICAL EFFECTS 



mainl}^ observational, not experimental, and bis discussion pbj^sical and 

 not to any considerable degree chemical. Literature on the subject is 

 meager. Some writers comment on the subject as quite obscure.'^ 



Concretions Defined 



Concretions are stones that grow, or, in other w^ords, are nodular 

 growths of various minerals sparsel}^ distributed through the countr}" 

 rock. They vary greatly in size, shape, composition, distribution, and 

 method of growth. 



They should be distinguished from secretions, which may sometimes 

 resemble them in form and structure. Such growths are formed in 

 cavities, and the outer layers are the older instead of the younger, as is 

 commonly the case in concretions. Geodes ma}^ be considered as im- 

 perfectly filled secretions. Though often externally resembling concre- 

 tions they differ from most of the latter b}^ being hollow, and from those 

 concretions which are cracked within, b}^ having a simple cavity bounded 

 by a shell of nearly equal thickness throughout. 



Concretions differ from stalactites, either pendant or coralloidal, which 

 are formed in the former case by trickling water, and in the latter case 

 by oozing water; they also differ from stalagmites which result from 

 dropping water ; and from other forms of cave deposits, as these 

 are all found with free surfaces and not embedded,- as is always the case 

 with concretions. Moreover, though all grow mainly b}^ additions to 

 the outside, like concretions, and though portions of them may resemble 

 concretions in form, yet the}^ are always attached b}^ a base to countr}^ 

 rock, which is never true of concretions. 



Concretions and Crystals 



Cr3^stals are the normal form which minerals take when solidifying 

 from solution or fusion without the interference of surrounding material 

 or contact with each other. Concretions result when the embedding rock 

 prevents the orderly arrangement of the molecules, though the}' may 

 still cohere in an irregular way. Evidence for this conception is found 

 in the numerous gradations observed between the typical extreme cases. 

 This is shown by numbers 8-10 and 17-21, plate 49, and numbers 5 

 and 7, plate 50. 



♦Furthermore, the discussion is limited to structures formed from aqueous solution and does 

 not treat to any considerable extent of those formed through the medium of liquids of fusion. 

 Quite possibly similar physical relations may occur in both, and consequently sometimes pro- 

 cesses and structures appear in the latter case similar to those we describe in aqueous deposits. 

 Lithoph3'S8e and other concretionary structures found in igneous rocks would seem to corroborate 

 such a view, but we have not had opportunity to study such forms. 



