THE PHYSICAL ORIGIN OF CERTAIN CONCRETIONS' 



JAMES H. GARDNER 



The literature on the subject of concretions is somewhat Hmited 

 in extent, and consists largely of descriptive rather than theoretical 

 matter. It is safe to say, however, that distinct types present different 

 problems for solution, and have resulted from divers combinations of 

 chemical and physical laws. The forces brought to play in the form- 

 ing of one kind may have played no part in the creation of another. 

 Types vary to such a degree that a valid classification is difficult to 

 prepare. Certain writers have made general classifications with 

 reference to manner of growth; for instance, Dana^ employs the 

 terms "centrifugal" and "centripetal" concretions for growths to 

 and from a center respectively. The latter includes principally 

 concretions of a geodal character. In a similar way the terms " excre- 

 tions" and "incretions" have been used.^ 



There can be no doubt as to the occurrence of these two general 

 types, but it has been supposed, in many cases, that concretions 

 have originated only through chemical phenomena. There are 

 exceptions, however, in which certain forms of rounded nodules 

 have been considered as resultant forms of physical forces. Kindle'^ 

 accounts for certain concretions of the Chemung by pressure of rising 

 gases of organic origin beneath impervious strata in a semi-plastic 

 state. This idea was suggested by observation of Agassiz and Horsford 

 on "raised hemispherical surfaces" in clayey mud near Cambridge.^ 

 Kindle makes use of this theory to account for a band of undistorted 

 fossils along the vertical and lower horizontal surfaces of certain of 



1 Published by permission of the Director of the U. S. Geological Survey. 



2 J. D. Dana, Manual of Geology, 4th ed., p. 98. 



3 J. E. Todd, "Concretions and Their Geological Effects," Bulletin 0} the Geo- 

 logical Society of America, Vol. XIV, p. 361. 



4 E. M. Kindle, "Concretions in Chemung of Southern New York," American 

 Geology, June, 1904. 



5 American Association for the Advancement of Science, Vol. IV, p. 12. 



452 



