PHYSICAL ORIGIN OF CERTAIN CONCRETIONS 457 



Unfortunately the writer has no quantitative analysis of the speci- 

 mens from the Chaco. They effervesce, however, in the presence of 

 acids. One would expect them to contain a high percentage of 

 soluble materials as is common to ordinary concretions. The last 

 material laid down physically in the water volume is necessarily of a 

 very fine character; furthermore, the water itself, being rapidly con- 

 centrated to the point of super-saturation, is forced to throw down 

 its minerals in solution at the same time the concretions are being 

 formed. If the water gains its clayey substances from ferruginous 

 beds, iron oxides will prevail in the last stages of the concentrated 

 solution. If calcareous, then lime will be prevalent in the fine silt 

 at the time when conditions are favorable for the forming of concre- 

 tions. The same will hold true in the case of other soluble minerals. 

 It is reasonable to suppose that, in accordance with the theory of 

 physical origin, most concretions of this class would be calcareous, 

 since lime is most common. Hence it is seen that while the compo- 

 sition of a concretion so formed depends on chemical relationship, 

 yet the concretionary process is itself a physical one. 



Concretions often show flattened or discoid shapes with the greater 

 axes parallel to the bedding planes of the containing strata. Writers 

 have suggested that this is due to there being less resistance to growth 

 in the horizontal than in the vertical planes. In some cases the strata 

 seem to have been pushed away by the enlarging concretion. Such 

 a flattening, however, may have been due entirely to pressure and 

 the strata pushed back around the concretion through resistance to 

 that pressure. There would also be a tendency toward development 

 of cleavage in the concretion at right angles to the pressure and these 

 planes might easily be confused with planes of stratification, the two 

 in normal instances being parallel. 



So far as the writer knows, attention was first called by Dr. George 

 P. Merrill' to the balling tendency of mud under artificial conditions. 

 He cites the phenomenon of concretionary balls having been formed 

 in mud flowing quietly from the mouth of an iron pipe; the instance 

 was that of pumping sediment from the bottom of the Potomac a 

 few years ago for the purpose of deepening the channel and filling 

 the so-called Potomac flats on the river front at Washington City. 



I G. P. Merrill, Rocks, Rock Weathering, and Soils, p. 37. 



