356 J. E. TODD COXCRETIOXS AND THEIR GEOLOGICAL EFFECTS 



ring-like. When these various forms are near enough to coalesce, very 

 complex and often surprisingly imitative forms appear — tubers, riveted 

 plate, and grotesque imitations of fruits, roots, parts of animals, or 

 various utensils. These are frequentl}^ collected in amateur museums. 

 When large they may be looked upon as of almost miraculous origin 

 and become objects of superstition. 



In loams, especiall}^ in connection with root marks, as explained later, 

 concretions are often vertically elongated, so as to resemble tubers and 

 fusiform roots. Again, some concretions, for some reason not yet satis- 

 factorily explained, are ver}^ much elongated horizontally. In the 

 Laramie formation the}^ attain size and shape fitl}^ described as log- like 

 (see plate 51, figure 1). They have been found over 100 feet in length, 

 counting the divisions separated by cross-joints. The detached sections 

 not infrequently closely resemble in shape, size, and color saw-logs 

 stripped of their bark. It has been suggested that they have formed 

 around organic matter collected in furrows on the beaches of lakes.* 

 This opinion is strengthened by their often occurring parallel with one 

 another, and sometimes so near as to coalesce, as in plate 51, figure 2. 

 Moreover, those on about the same level have been observed to lie in 

 systems, extending for many rods, or even miles, perhaps, in broadly 

 sinuous lines, like a lake shore. 



Location 



Concretions, like cr3^stals, seem to owe their location to predetermining 

 circumstances. In the case of the latter a particle of similar substance 

 accidentally present seems to act as a starting point or nucleus. A 

 particle of precipitating agent may also act in a similar wa3^ Bones, 

 leaves, shells, etcetera, are found embedded by the concretions which 

 have formed around them ; but where no such nuclei are present the 

 precipitating mineral tends to concentrate itself at nearly equidistant 

 intervals. If the precipitating influence is at high temperature and pro- 

 gressing rapidl}^ we may expect these centers will be nearer together 

 and the resulting concretions smaller. 



The foregoing remarks ap})ly to spaces fully submerged in ground 

 water or far removed from the surface of ground water. It is probable, 

 however, as will appear from consideration given in a subsequent sec- 

 tion, that the surface of the ground water, or water-table, is preeminently 

 a zone of concretionary growths. There the various precipitating influ- 

 ences are especially active, and by tiie fluctuations of the surface of the 

 ground water this action is intermittent, which fact sometimes increases 



* J. E. Todd : American Geologist, vol. xvii, 1896, p. 347. 



