GEOLOGICAL EFFECTS 363 



Their usual size and shape resemble quite closely a common lead-pencil. 

 Their method of growth, as given above, is a matter of inference rather 

 than positive observation. If those described in the earlier part of this 

 section may be properly called excretions, because the particles move 

 outward to their final deposition, then these may for a similar reason 

 be called incretions ; but neither term is wholly satisfactory. 



Incretions are often found in the loess composed also of carbonate of 

 lime. They are often like clay pipe stems in form and size. Moreover, 

 they may at certain levels increase by accretion or intercretions into 

 nodules of considerable size, resembling tuberous roots in form and in 

 relation to the stem-like concretions, as is shown in figure 22a, plate 

 49. Such tuber-like enlargements seem more likely to occur near 

 levels where the top of the ground water stands for some time. 



Geological Effects 



in general 



Concretions may be considered as a kind of local cementation of un- 

 consolidated strata, and therefore may merge gradually into wholly con- 

 solidated strata, which are well known to form the controlling factors of 

 topography. Hence we are prepared to believe, on general principles, 

 that concretions themselves may be very influential in topography. 



TEEPEE BUTTES 



This striking feature in certain localities on the western plains was 

 ingeniously traced by Mr G. K. Gilbert some years ago to concretions 

 cementing together colonies of Lucina occidentalis.^ All gradations may 

 be pointed out along a certain horizon of the Pierre formation from the 

 sharp, conical knoll to the low mound with the concretions scattered 

 about on the surface. Near the trail between Dead wood and Bismarck 

 the axis of one of these buttes stands completely uncovered in the form 

 of three or four huge boulder-like concretions, each of many tons weight, 

 resting one on another, as though giant hands had piled them there. 

 The relations of these concretions to the teepee buttes is clearly shown in 

 figure 23, plate 49. 



BAD LAND PINNACLES 



The scenery of the White River bad lands has been long noted for its 

 grotesque and picturesque features. Pinnacles, castellated shapes, pyra- 

 mids, pillars, spires, etcetera, are found in unlimited numbers. The 

 prevalent deposit is a sandy marl, traversed with irregular strata of sand 

 and gravel. Both in the marl and in the sand many flat concretions have 



* Seventeenth Ann. Rept. U. S. Geological Survey, part ii, p. 569. 



