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



Solutions of — 



By- 





Become solid — 



CaCOs, 



H2CO3 



Evaporation of COj 





CaCOs (calcite). 



FeCOs, 



H2CO3 



a 



" CO2 

 Action of H2S 

 " " CaCOs 

 ** " oxygen 





FeCOg (siderite). 



FeSa (pyrite). 



FesOs, 3H2O (limonite) 



FeSO, 





(. u 





K (i (i 



(( 





" ' " organic matter 



FeS^ 



Na^SiO. 



5 



" " CaCOg, or 



CaSO, 



SiO^ (quartz). 



Any substance 



Drying or cooling 





The same. 



Methods of Growth 



ACCRETIONS 



Xbe common conception of the growth of a concretion, as is perhaps 

 suogested by the name, is that particles are gathered together toward the 

 center so as to produce growth regularly and steadil}^ from the center 

 outward. Such may be aptly called an accretion. Such accretions, from 

 the nature of the case, will be solid and include or enmesh particles of 

 the rock in which they form without any considerable disturbance of 

 them. It is conceivable that the molecular attraction between the mole- 

 cules of the concrescent minerals may be so strong as to exclude foreign 

 particles to some extent, but evidence of such action has not been often 

 observed. That carbonate of lime is not likely to do so with common 

 sand, seems attested by the occasional formation of regular calcite 

 crystals with true angles and planes in sand, which include the sand 

 grains lying within the space of the crystal without any trace of exclu- 

 sion. As crystal growth is without doubt by simple accretion, it follows 

 easily that calcareous accretions will not exclude sand, even if they may 

 some other foreign particles, and it seems probable that other concre- 

 tion forming minerals act in a similar way in similar circumstances. 



Simple accretions in stratified material preserve the stratification in 

 their substance. Accretions may also have a radiate structure, which 

 may be due to the development of a modified crystalline structure. 

 They ma}^ probably also show rhythmical or concentrically banded 

 structure as in figure 12, plate 49. This is not certainly known but 

 probable. It may be due to conditions corresponding to those which 

 form specter and concentrically stratified crystals — that is, to intermit- 

 tent periods of deposisition with possible-counter influences between, 

 or to the intermittent deposition of impurities or of some coloring 

 matter. 



While it can not be questioned that they are normall}^ solid, it is con- 

 ceivable that sometimes a wet mass of calcium hydrate or of calcareous 



