CONCRETIONS — BASSLER 323 



strata, wliile those in the surrounding strata may be crushed by this 

 weight. For this reason solid concretions occurring in shaly beds, 

 such as those in the Cretaceous shales of the Great Plains, yield 

 excellent fossils. 



Certain concretions displace the rocks in which they are contained, 

 indicating that they were formed after the deposition of the enclos- 

 ing rock, pushing aside the rock in the course of their growth. 

 Such curvature of the enclosing rock formation occurs most fre- 

 quently in shales and is absent in sandstones, giving rise to another 

 possible explanation, namely, that the shale beds may have settled 

 around the concretion. Another method of formation is shown in 

 the case of siliceous concretions exposed when limestone wears away. 

 Here, it may be observed that as the limestone was dissolved by sur- 

 face waters, the siliceous impurities segregated into nodules which 

 were left at the surface or along water channels in the characteristic 

 globular forms. 



Deep-sea dredging in the red clays of the ocean depths often brings 

 up concretions in the form of nodules of manganese dioxide. Col- 

 loidal silica, the gelatinous form, is, next to calcium carbonate, the 

 most abundant of the soluble substances carried to the sea by the 

 rivers. However, sea water contains but a very small percentage of 

 silica, and therefore the large amount of gelatinous silica must have 

 been deposited upon the ocean bottom. Here it assumes the nodular 

 form by segregation, and concretions are the result. Such condi- 

 tions of sedimentation occur over great stretches of sea bottom, so 

 that today, as well as in the past, concretions occur usually in defi- 

 nite beds that can be traced over many miles. Thus, they can be 

 used for correlating strata of the same age over wide areas. 



Concretions are commonly composed of a single mineral, but 

 frequently other substances occur as impurities. Calcium carbonate, 

 silica, iron oxide, hydroxide, carbonate, or sulphide are the most 

 common component materials. Clay stones and the calcareous con- 

 cretions in shales and sandstones are composed largely of calcite, 

 as are the concretions so abundant in the loess and the glacial clays 

 widespread over the northern United States. To these latter forms 

 the Germans have applied the name " Loesspiippchen ", or " loess 

 dolls" (pi. 1, fig. 4). Ironstone concretions are most common in 

 sandstones and consist largely of quartz grains cemented by the 

 oxide or hydroxide of iron. Such concretions may be collected in 

 modern lakes and, indeed, are forming today in certain soils. Con- 

 cretions of pyrite occur in dark shales containing organic matter, 

 sometimes in sufficient abundance to constitute a source for this ore. 

 Barite, or heavy spar, forms the beautiful concretions known as 

 " petrified roses ", in sediments accumulated under arid conditions. 



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