456 JAMES H. GARDNER 



is a very common occurrence among concretions from sedimentary clay 

 and shale beds. Or it is quite possible that a concretion formed 

 as above may be subjected to stronger currents or clearer water and 

 be eroded to any imaginable shape with smooth outlines. It may be 

 carried to a distance and incorporated in a sediment of an entirely 

 different character from that in which it had its origin. Its composi- 

 tion as a whole would likely in most instances vary from the material 

 immediately surrounding it. 



The following are analyses of a typical clay and inclosed concre- 

 tion from the Champlain clays of the Connecticut Valley.^ 



DARK CLAY LAYER INCLOSED CONCRETION 



Silica 51 



Iron oxide 8 



Alumina 20 



Lime 



Magnesia i 



Manganese oxide 



Carbon dioxide 



90 Silica 42 -93 



81 Iron oxide 13-66 



43 Alumina 25 . 49 



97 Lime 3 .07 



27 Magnesia 2 .09 



9 ^ Manganese oxide i . 10 



30 Carbon dioxide 18 



It will be observed that the silica and carbon dioxide of the concre- 

 tion are somewhat lower than that of the clay, while the remaining 

 constituents are higher. While the percentage of lime is 2.10 per 

 cent, greater than in the clay, yet it is not in sufficient amount to justify 

 the term "lime-concretion." The quantity of Hme present in this 

 type may vary from a small amount to more than 50 per cent.^ From 

 the presence of a high percentage of lime it does not follow that this 

 constituent has been the prime factor in the origin of the concretion. 

 Concretions of the character under discussion often show structure 

 made up of very fine material such as would have resulted had they 

 been formed from the adhering of minute particles. Clay and shale 

 concretions have been known to contain gravels, coarse sand grains, 

 or small organic relics as nuclei. Some show small pebbles and coarse 

 sand grains in the form of interior concentric shells. ^ Similar struc- 

 ture has been pointed out as occurring among the concretions which 

 form in the bed of the Rio Chaco. 



1 J. M. A. Sheldon, The Champlain Clays of the Connecticut Valley. 



2 C. B. Adams, "Concretions," Second Annual Report on Geology of the State of 

 Vermont, pp. 111-18. 



3 "Champlain Clays," loc. cit. 



