764 PRINCIPLES OF STRATIGRAPHY 



while the smaller ones commonly have a goniatite shell at the center. 

 Frequently the stratification of the enclosing beds on either side 

 appears to be continuous through the concretion, while the beds 

 above and below are arched, owing to the pressure caused by the 

 growing septarium. Some of this arching may be due to the com- 

 pression of the shale around the resistant septarium. 



Calcareous clay stones are also common in the loess, where they 

 are known as Losspuppen or Lossmannchen, by the Germans, as 

 Fairy stones by the Scots, and as Imatra stones by the Finlanders. 

 They form in parallel lines, giving the deposit the appearance of 

 stratification, though the original bedding may have been quite 

 diverse. The lines of concretions mark rather the successive levels 

 of ground water than any structural features of the rock itself. 



Claystone concretions of very regular form are found in the 

 Champlain clays of the Connecticut A'alley. while clay-iron-stones 

 enclosing ferns, insects, crustacean and other remains, occur in the 

 Carbonic shales of Grundy County, Illinois, the fossil-bearing con- 

 cretions of Alazon Creek having become famous on account of their 

 well-preserved organic femains. 



A special form of calcareous concretion is known from the black 

 Devonic shales of Michigan and Ontario, being especially abundant 

 at Kettle Point, Lake Huron. These are spherical, or nearly so, 

 and are composed of radiating crystals of calcium carbonate, which, 

 growing outward, crowd the enclosing strata until they curve about 

 the concretion. The structure resembles that of a fibrous wood, 

 and fragments are not infrequently mistaken for "petrified" wood. 

 Their manner of occurrence, growih and significance is fully dis- 

 cussed by Daly (11). 



Concretions of iron pyrite and ©f marcasite are forming in many 

 strata. Sometimes these are globular masses composed of crystals 

 of pyrite. at other times they have a radial structure. Generally 

 some object of organic origin forms the nucleus around which the 

 concretion grows. In the Cretacic clays of New Jersey pyrite con- 

 cretions are forming aroimd fragments of lignite, all stages of in- 

 crustation and replacement being observable. 



Siliceous concretions are common in calcareous formations. 

 Thus flints characterize chalk, and chert layers abound in many 

 limestones. Flints may occur in continuous layers in massive chalk 

 beds in which their arrangement alone indicates stratification. As 

 in the case of the concretions in the loess, these lines of flints need 

 not have a necessary relation to the original stratification. 



In many cases, however,, the flints formed around a siliceous 

 sponge or some other organi^n which acted as a nucleus to attract 



