CONCRETIONS 



719 



and "Lossmannchen," of the German loess, the "Rlarlekor" and 

 "Nakkebrod" of similar Swedish deposits, the "Kankar" of India, 

 and the clay iron stones of the British Carbonic shales are other 

 examples of concretions of this type. 



According to their method of growth concretions have been 

 divided by Todd into four types (20) : 



1. Accretions. 



2. Intercretions. 



3. Excretions. 



4. Incretions. 



Accretions grow regularly and steadily from the center out- 

 ward by successive additions of materials. This type of concre- 



FiG. 143. Concretion (accretion) of clay stone, Connecticut Valley. (After 

 Gratacap.) 



tion will be solid from the center and if of subsequent growth will 

 include or enmesh particles of the rock in which it forms without 

 any considerable disturbance of it. The stratified structure may 

 also be preserved in accretions, unless by crystallization a radiate 

 structure is produced. Examples of this type are the remarkable 

 concretions of the postglacial clays of the Connecticut Valley. 

 (Sheldon-19.) (Fig. 143.) 



The radial structure due to crystallization is well developed in 

 the large spherical concretions of the Devonic black shales of Kettle 

 Point,* Ontario, and of Michigan. From their fibrous structure 

 they are riot infrequently mistaken for petrified wood. (Daly-5.) 



Intercretions grow by accretion on the exterior and by inter- 

 stitial addition, causing a circumferential expansion and resultant 

 cracking and wedging apart of the interior of the concretion. The 

 cracks. widen toward the center and are commonly filled by mineral 



* So named from the resemblance of partly exposed concretions to inverted 

 kettles. 



